If you're looking for reasonably priced 22 mesh canvas or silk gauze for petit point, this store has the best prices for these items. Look under "unpainted canvas." Shipping is free if you purchase $35 or more; $4.95 for orders less than $35. If you sign up for the newsletters, you will get notification of periodic 20% discount specials, which decrease the price even further.
Of course, to get these good prices, you need to buy in fairly large quantities (1/4 yd or more). If you only want to buy small amounts, check out ebay, microstitchery.com, janet granger's website, felicity price miniatures (mesh canvas only, she calls it "etamine"), and natalia frank ("scarlet sails miniatures").