I'm just starting out. Anything other than aluminum cans would be fine. I have a lot of things in-work and casting is not quite on the radar yet. My wife gets annoyed when I have too many things going on (we're short on space).
I've been doing a lot of reading on casting. The cans would work if they had no paint and no plastic coating on the inside. I saw one article where a person cut them into tops, bottoms, and split sides, but that's way too much work to get some good working material. I'm planning to ask the guys at the local recycling place if I can scavenge some junk out of old PCs and printers. If they let me, there's probably some nice aluminum to be had, along with stepper motors and machined rods and such.