From
[3.x] Shax's Indispensible Haversack (Equipment Handbook) - Giant in the Playground Forums
"Auran Mask
Price: 60 GP
Weight: --
(Complete Mage p. 134)
Provides a +5 circumstance bonus against inhaled toxins (which stacks with Antitoxin), but this is easily overshadowed by 10 minutes of water breathing. Requires less upkeep than Air Plants. For longer durations, consider a potion of water breathing for 750 GP, which lasts for 10 hours.
Craft (Alchemy) DC: 25"
Sandstorm also has the Filter Mask for 1 gp. It works for up to 4 hours (can be reused, just need to do the laundry; so buy several of them like you would underwear). +2 on saves against gas-based effects and immunity to suffocation from dust and sand and immunity to the effects of magical dust (hurray, 100% immunity to the most broken item in the game, the Dust of Sneezing and Choking!)
Further, Sandstorm has the Mask of Sweet Air wondrous item, but it's grossly overpriced. For 147,000 gp, you get complete immunity to airborne stench and poisons and continuous water breathing.
EDIT: MIC has an updated Mask of Sweet Air that's only 2000 gp but also merely gives a +5 on saves instead of immunity.
Honestly, since there's lots of cheap stackable bonuses you can get but immunity is so expensive, you might be best off just getting a gigantic save bonus and taking the
Steadfast Determination feat so you can't fail fort saves on a 1. Basically the same thing as immunity.