where can i buy spray paint?
you can get paint at any general store such as wal-mart or your local hardware store. You may run into an age issue, and if so, then you are going to have to have somebody buy it for you.
how do i get into doing art for skateboard decks, for skateboard companies?
Get in good with a Pro Skater, they are the ones who pick out the art for their decks.
Good Skateboard companys?
ok for decks the best are almost and habitat for wheels bones for bearings reds for trucks independents or grindkings for griptape black magic