slm
Formerly nextreme
I re-use commercial spools. Got a huge box of discarded ones from a photo lab. There's a small piece of film sticking out of each spool after their machine cuts it off from the roll. I just splice the bulk film on with a piece of Scotch tape (the "frosted" kind). Never have had an issue and they were free!
I do the same, got a bunch of old cartridges from my local Costco, back when they used to process film. What a drag they stopped, it was very convenient.