i want to use foam in my hang on back filter. i dont need the carbon inserts they sell cuz i got enuff in the canisters. anyone know a cheap alternative to a pet store for the foam?

Check a pond or farm store. Most of them sell it by the foot. Tractor Supply and Agway for sure have both a coarse and a fine.

Some of the bigger fabric stores also have foam (Joann's for sure) you just need to make sure it's open cell foam. . Batting and polyfil also work in a pinch but polyfil clogs up pretty fast.

Foam for a HOB wont cost much $5 bucks depending on the filter you are buying for and will last forever.

I found the offical brands of fish filter foam rot and fall apart after a while. One of the reasons I switched to the pond types. They last so far a very very long time. I have a few that have been in use for about 5 years now.