Local purchase: Canadian Tire, 40-packs of Crosman Powerlet for about $30. (~$0.75@)

Don't skimp on quality. Some brands have inconsistent fills and cartridge lengths. Avoid JT, Brass Eagle Brand and anything unbranded.

Best if you want to order in bulk: Leland brand in tubs of 450. Leland supplies 12-gram powerlets to the US Military for the inflators on life vests so they are quite consistent.

with a blow back gun probably around 100

Expect between 75-100 rounds out of one CO2 powerlet. If it's a full-auto gun, you'll get a lot of cooldown and you'll get fewer shots. For example, out of my TF11, I can get about 120 shots in semi firing slowly. With short, controlled 3-round bursts, I can get about 100 rounds. I'm lucky to get 75 if I spray in full auto.