Wis to AC, in light armor or in any armor
Two levels of either moon-warded ranger from Dragon vol. 340 or swordsage from Tome of Battle gets you Wis-to-AC even while wearing light armor. Saints, from Book of Exalted Deeds, get Wis-to-AC in any armor, but saints are overpowered. Better balanced, and possibly an excellent choice for a paladin/ardent, the argent fist prestige class from Faiths of Eberron also gets Wis-to-AC even in armor, and the Monastic training feat it requires 1. doesn’t actually require levels in monk, and 2. is a requirement for the excellent Tashalatora feat from Secrets of Sarlona, which can give your ardent levels most of the benefits of monk levels. You’ll need evasion from somewhere, though—a ring of evasion would work but that delays things until higher level due to its price tag.
Cha-to-AC, or Wis-to-paladin-abilities
The Ascetic Mage feat from Complete Adventurer can turn Wis-to-AC, from any source, into Cha-to-AC, but that requires 2nd-level arcane spells somehow. Alternatively, Serenity from Dragon Compendium changes all paladin class features that use Charisma to instead use Wisdom. This is vastly superior, particularly for a paladin/ardent.
Con to AC
With 20 Constitution, the deepwarden prestige class from Races of Stone or the fist of the forest prestige class from Complete Champion warrant a look, just because they add Con to AC.
Deepwarden requires that you be a dwarf, as well as the terrible Endurance feat, and then takes two levels to grant this feature, but it stacks with armor. The big problem with deepwarden is that it gets little-to-nothing else.
Fist of the Forest requires three feats (Great Fortitude, Improved Unarmed Strike, and Power Attack), but one of those (Power Attack) is at least pretty good and can even be substituted for another solid option (Stone Power from Tome of Battle). And Improved Unarmed Strike can be gained by taking levels of unarmed swordsage (an official adaptation suggestion in Tome of Battle), or by going with Tashalatora. The big problem with fist of the forest is that it is incompatible with armor. Still, combined with one of the Wis-to-AC options, that could be quite good. Even monk could be good at that point, since you’d get it in one level and couldn’t use light armor anyway.
Shield bonus to touch AC
Lords of Madness has the Parrying Shield feat, which if you are using a shield, could be decent. The hide shield from Sandstorm offers a base +3 bonus to AC, has the total cover option of a tower shield, and doesn’t require special proficiency or inflict massive penalties the way tower shields do. Still, shields are painful, because two-handed weapons are so good. A +1 animated hide shield isn’t terribly pricey, though, and could be a very good solution to that problem.
Armor bonus to touch AC
Finally, the Deflective Armor feat from Races of Stone allows you to apply your armor bonus to AC against touch attacks while psionically focused. It does require the otherwise-mediocre Heavy Armor Optimization feat from the same source, but two feats might be worth it.
My feeling on AC is that you should invest very minimally in it. Touch AC is absolutely more valuable, and possibly worth another look, but always keep in mind that the majority of AC options are overpriced. I do not, for example, recommend higher enhancement bonuses on your armor or shield, even if those apply against touch attacks. I also don’t recommend a ring of protection—and if you get one, I strongly recommend keeping it at ring of protection +1 and going no higher.
But there are some reasonably-priced options here. Serenity is a fantastic feat no matter what, so that one’s almost a gimme. Taking Monastic Training and Tashalatora then gives you Wis-to-AC, so you could easily go with a simple 2nd-level paladin/18th-level ardent build, and get very good touch AC just by having excellent Wisdom. Not worth two feats, but Tashalatora does a lot more for you because it also gives you a monk’s unarmed strike feature, fast movement feature, and flurry of blows feature, scaling with your ardent levels. If you go that route, know that Intuitive Attack from Book of Exalted Deeds will get you Wis-to-attack, too, so you can really focus on Wisdom—adding it to attack, AC, and all saving throws.
Alternatively, especially if your ability scores are locked in and you cannot switch into a hard Wisdom focus, you could spend the same three feats on Heavy Armor Optimization, Deflective Armor, and Parrying Shield. The upside is you can use armor, and with a +1 animated hide shield you don’t even sacrifice much offense. The downside is that you don’t get the SAD benefits of Serenity, and you don’t get flurry of blows as you would with Tashalatora. But +14 touch AC from +1 full plate and +1 animated hide shield is going to be hard to beat even for a Wis-SAD build, so it’s probably better at the specific thing you’re asking about.
The other benefit of Heavy Armor Optimization, Deflective Armor, and Parrying Shield is that Deflective Armor is a psionic feat, and the others are fighter bonus feats. That means two levels of psychic warrior can get you two of the three as bonus feats, while also giving you power points that stack with your ardent points, as well as potentially a couple of psychic warrior powers that aren’t easily available for ardents (expansion and vigor, most likely). Practiced Manifester (ardent), which you should be taking anyway, means that you can also afford two levels of paladin and two levels of psychic warrior without disrupting your ardent manifesting too much.
Deepwarden and fist of the forest, to me, don’t add enough. Fist of the forest wants you focusing on unarmed strikes and forces you to be unarmored, while you’re probably better off with a two-handed reach weapon, and in any event it requires a ton of feats. It kind of synergizes with Tashalatora, but levels in fist of the forest aren’t ardent levels and therefore don’t progress your monk features. Deepwarden is race-restricted, and while dwarves are a fine race, they’re not necessarily ideal—and spending two whole levels on Con-to-AC is a big cost, because seriously, that class has almost nothing else. By the same token, moon-warded ranger or swordsage seem too pricey—though both of those, at least, also do other things for you, unlike deepwarden. But all of these are coming at the cost of ardent manifesting, and while we’re mostly talking about two levels—so that alongside the two paladin levels, we’re still covered by Practiced Manifester—it’s still a cost, particularly to your power points.