A dragon that's a suitable foe for a group of typical PCs that have ready access to the 7th-level Sor/Wiz spell hide from dragons [abjur] (Spell Compendium 114) is Challenge Rating (CR) 13 (i.e. 13 is also the minimum typical wizard level needed to cast the hide from dragons spell). While CR is by any account an imperfect measure, a starting point remains necessary, and in this regard a young adult red dragon (Monster Manual 75–7) makes a suitable example.
A young adult red dragon has triple standard treasure for its CR 13. This is a lot of treasure. (Go ahead—try it yourself. Ten tries gave me anywhere from 15,000 to 85,000 gp.) The young adult red dragon has 19 Hit Dice therefore 7 feats.
Were this DM deeply concerned about limiting the efficacy of the hide from dragons spell for the least resources, the feat Technomagic Implant (Dragon #351 93) and the tremorsense disk (4,000 gp; 0 lbs.) is my pick.1 For one feat, the dragon's feet magic item slot, and a few grand in gp, the dragon gets tremorsense 20 ft., a sense that isn't explicitly or implicitly neutralized by the hide from dragons spell. This is—I guess—a decent investment for limited immunity to the hide from dragons spell: The dragon lands to hunt, detects everything touching the ground in a 20-ft. radius, and flies off if its tremorsense detects creatures it can't see. (Drachenflagrion, the sample young adult red dragon from the Draconomicon (256) has among his feats the feat Cleave (Player's Handbook 92), which would be the first to go in this DM's campaigns.)2
Alternatively, getting blindsight (that's also not explicitly impaired via the hide from dragons spell but may be implicitly impaired if it relies on the dragon's hearing, sight, or smell) is easier but I suspect will still be viewed as more resource intensive:
- 1 feat: The feat Favored in Guild (Church of Shar (City of Splendors: Waterdeep 45–6)) (Complete Psionic 151) grants, in this case, the favored one the ability to take a standard action to gain for 1 min. blindsight 30 ft. The favored one can do this 1/day per level hence HD hence 19 times for a young adult red dragon. That's plenty if the dragon activates it before swooping in and killing something, but the dragon must worship Shar and be a well-respected and vital part of her local church. This seems out of character for the dragon in question. An argument can also be made that this blindsight is based on the creature's sight.
- 2 feats: To get blindsight 20 ft. for 5 rounds 2/day takes the feats Devil's Favor then Devil's Sight (Fiendish Codex 2 81 and 83, respectively), but this also mandates a pact with a devil, which also seems out of character for the dragon in question. In addition, an even stronger argument can be made here that this blindsight is based on the creature's sight.
- 3 feats: The feats Shape Soulmeld (shadow mantle (Magic of Incarnum 85)) (40) then Open Lesser Chakra (shoulders) (ibid.) and an appropriate incarnum feat that offers 1 or more points of essentia like the feat Bonus Essentia (35) allows the meldshaper to invest in the mantle essentia to so that the meldshaper can take a swift action to activate or deactivate both a 0-ft.-radius (+5 ft. per invested essentia) of magical darkness that also renders inoperable blindsense and that's centered on the meldshaper and blindsight 0 ft. (+5 ft. per invested essentia) for the medlshaper.
- 4 feats: The feat Combat Awareness (Player's Handbook 2 86–7) has serious prerequisites and yields, in this case, a comparatively pitiful blindsight 5 ft., but the effect is constant effect.
The DM would typically view 3 feats as too many, but, the magical darkness effect offered by the shadow mantle soulmeld provides the dragon with further protection from the surprise attacks that the hidden dragon hunters will launch, which is a good thing—not, in this player's opinion, three feats' worth of good thing, but worth the GM's consideration in this case nonetheless. (Drachenflagrion, for instance, could probably lose the feats Cleave, Improved Snatch (Dr 71), and Snatch (MM 304), and only he and the occasional pony would notice.)3
Using only magic items
Most magic items that grant special senses are too expensive or limited in their scope—you can check in the section Special Senses here to confirm. The boots of tremorsense (Magic Item Compendium 79) (5,000 gp; 1 lb.) may be an exception—the dragon can simply activate them before landing. Again, though, this dragon sounds as if it may view donning footwear as undignified.
Just imagine this dinosaur red and with wings, okay?
…Or just don't worry about the hide from dragons spell
The hide from dragons spell has a duration of only 10 min. per level but also lasts only until "a warded character touches or attacks a dragon or the dragon’s hoard" whereupon "the spell ends for all recipients." In other words, the dragon hunters will only have a few hours each day of undetectable dragon hunting. (The hide from dragons spell is, after all, like the spells limited wish and simulacrum, a 7th-level spell, not to be tossed around lightly!) If the dragon gets sloppy by not changing its hunting grounds and hunting times in the face of dragon hunters, the advantage of the hide from dragons spell is that the hunters—if they're lucky and the dragon doesn't recognize telltale signs of their ambush—will act in the surprise round (and, obviously, the dragon won't), and the first attack a hunter launches at dragon ends the hide from dragons spell for everyone.
Now, this DM is the first to admit that the surprise round is incredibly important in D&D 3.5, especially since a level 13 wizard's hide from dragons spell means up to six attacks in that surprise round, if not more. However, a dragon should be able to take a surprise round's worth of attacks from such a group then either flee or engage, as is its wont. (If the dragon can't survive the surprise round and the first round's worth of attacks from the dragon hunters then I suspect the dragon hunters are extremely well prepared, the dragon is ill-prepared, or both!)
Still, the dragon should be smart beforehand, employing the 2nd-level Sor/Wiz spells blur [illus] (PH 206) and scintillating scales [abjur] (SpC 181) or even finding a way to employ the 4th-level Sor/Wiz spell ray deflection [abjur] (SpC 166) to ward off touch attacks and ranged touch attacks (q.v. here). For a young adult red dragon, playing smart shouldn't be a problem: Its Intelligence score is 14. Further, almost any dragon should try to marshal such resources whether or not it expects dragon hunters; they're just good sense for a dragon.
Finally, if the dragon's primary reason for leaving its lair is food, this need is obviated by, for example, a simple ring of sustenance (Dungeon Master's Guide 233) (2,500 gp; 0 lbs.). Then the dragon hunters'll have to come to the dragon's lair or wait for the dragon to leave for a reason other than sustenance.4
1 Until another answer comes up with something better, obviously.
2 Having monsters take the feat Cleave has always struck this reader as the definition of overkill.
3 Of course, with his current feats Drachenflagrion need never land to hunt. Take that, dragon hunters.
4 While the dragon in question may be too proud, this GM would urge the dragon to acquire minions. Even frightened local commoners make a better spy network than the zero minions this dragon has now!