You're out of luck for Dodge as far as items go - the only item that grants it, to my knowledge, is the Manual of the Winding Way (Dungeon 117, p78) which is sadly an artifact.
Mobility is much easier - the Magic Item Compendium features the mobility enchantment, which is a +1 enchantment that can be added to any light armour and grants you the feat.
You can use the magic item guidelines to suss out what the price of an item that grants these feats would be, subject to DM approval.
Arms and Equipment Guide lists Mobility as a power intelligent items can possess, at a cost of 10,000gp (plus the cost of the intelligent item). Page 139.
General feat item
The same book also lists suggested prices for feats, on page 128. The recommended cost is 10k, plus 10k per prerequisite - so Dodge would cost 10,000gp and Mobility would cost 20,000gp.
Continuous items of spells
This is D&D, baby! Anything you want, spells can do, and there are guidelines for permanent items of spells. The formula is spell level * caster level * 2000gp * special modifier based on spell duration. For the spell you want (heroics) that modifier is 1.5. So the cost of each item (you would need one for each feat) is 2 * 3 * 2000 * 1.5, or 18,000gp. A typical 8th level character can't afford this, since wealth-by-level for level 8 is only 27,000gp.
There are alternative spells you can use - mirror move gives both feats too, and strategic charge gives Mobility - but their durations are shorter, so the cost is higher.
Forget the items, just use a spell
There are two ways to permanently swap out some of your feats you already have for these feats. Which one you want to use depends on what's more readily available in your setting - psions, or high-level wizards and clerics.
Get punched in the brain, pay some XP, re-pick feats you previously picked. The power is from the Expanded Psionics Handbook.
Embrace the Dark Chaos and Shun the Dark Chaos
You want to get these spells cast on you in succession. Embrace the dark chaos swaps out a feat you have for an Abyssal Heritor feat. It doesn't matter which, because shun the dark chaos trades an Abyssal Heritor feat for any feat. The spells are from Fiendish Codex I.
These spells are often used to cheese free feats (for example, trading out an elf's weapon proficiencies or the Alertness from your familiar) but as long as you're using the spells on feats you actually picked out in your feat slots, there's no reason to cry foul.
Other, worse ideas
- Kill yourself and get reincarnated until you become a Human or Strongheart Halfling, and thus get a bonus feat.
- Sell your soul to fiends or Elder Evils in exchange for bonus feats (Fiendish Codex II and Elder Evils, respectively)
- Use the retraining and rebuilding rules in Player's Handbook II to swap out levels for levels of Fighter.
- Contract a strain of lycanthropy, immediately giving you a bunch of Hit Dice, and therefore new feats.
- Chop off your leg and replace it with the Golden Dancing Pegleg from Dragon 318 (p54) for 55,000gp.