The affordable air polisher we use costs around $16 shipped on eBay.
Search for air polishers on ebay here. (affiliate link) There are a LOT of models that meet my keywords; be sure to pick the model pictured above. I haven't tested every single model, but of the three I tried, this was the easiest to aim. We don't air polish much due to the powder mess, but when you need it, you need it. At $16, you can afford to keep one or two in the drawer. At time of writing, there was a listing for $14.29 from a US seller. Least expensive 4 hole model I could find.
If you don't mind waiting longer for shipping, and not being able to return, here's a listing with five extra tips included.
A couple of tips on air polishing:
Keep dirty pennies around to test your polisher. It should clean a rusty penny instantly.
Turn the air pressure down - you only need about 25-30 psi of pressure on air line. Anything more than that can hurt.
Flush before and after every patient - keep a second powder jar. If you accidentally contaminate that second jar, you don't have to throw away the entire new bottle. After polishing, open the lid, dump the unused powder in there for the next patient. The after flush
Autoclave the tips only - the main body will warp and melt in the autoclave. Use barrier plastics and/or disinfectant wipes instead.
Avoid patients with extensive composites - baking soda will matte the finish. You can repolish them afterwards if you need to.
Try Glycine powder - it's a bit less abrasive and a lot less salty than sodium bicarbonate. 3M sells a Clinpro glycine powder for prophy polishing. It's safe to use around brackets or implants, and recommended for perio patients, too.
Some final eBay buying tips:
Buy from US sellers. Returning to China is basically impossible.
Pick the free shipping item with the lowest cost. Shipping cannot be refunded, so by picking the more expensive "free shipping" listings, you'll be more protected from lemons.
Buy in bulk when it makes sense - the combo I listed at the very beginning of the article seemed less expensive than buying the individual items. But a 5 or 10 pack may be cheaper still.
*at time of publication. Market prices fluctuate, of course.