Dixie Art lists a Paasche Tank/Propel Valve P/N 993B for $5.30 + S/H, but I encourage you to get a compressor. A reliable source of uninterrupted dry air is equally as important as an airbrush. Propel cans are good in a pinch, but are a very expensive way to buy air over time.
Personally, I use a garage compressor with a 3 gallon tank and pipe the air into my man cave, but most are so loud that they can't really be used indoors and keep peace with SWMBO. Also, most people don't have the opportunity to punch holes through exterior wall. If you can get away with it, garage compressors work fine and serve multiple purposes. I use a pressure regulator and water trap mounted on my spray booth, and have it wired to power the compressor remotely. I did a quick search and found that Lowes is offering a 2 gallon tanked Campbell & Hausfield compressor with brad nailer/stapler for $79! Otherwise expect to pay $150 or more. Remember, these are LOUD!
A dedicated hobby compressor can be found for around $120 and If you need a quieter solution, the Silentaire Scorpion I sells for for around $170 delivered (Bear Air & Dixie Art are both recommended). It's rated for 50db, which is impressively low for the type of compressor!
Another option is using a CO2 tank, which would be totally silent, other than the hiss of air from the airbrush. CO2 tanks can be fairly expenseive to buy, require refilling, and with my luck guaranteed to run out at night in the middle of a tricky camo paint job. Only to be considered if only complete silence is required.