Those are all great ideas! Thanks for sharing!
I had a coworker give me several garbage bags full of fabric, all kinds. I won't need to buy interfacing for a long time! I also got a bag full of thread from my great grandmother's stash after she died. It is definitely lasting.
Another great idea is looking for wholesale supplies on places like ebay. A few years ago I got like 200 zippers for like $24 shipped or something? I still have a ton left.