Where to get cheap fish?

Hi, I'm thinking of getting back to my old hobby by keeping fish . Can be said that I need to start from beginning. So need advise of what type of fish to keep and where can I get them where cost is relatively cheap .
I was thinking to start with a small tank. Have been looking ard of a small tank with filter inbuilt. The size of the tank ard 30cm x 30cm. So may be the xpert here can give me advise.