I work at a gardening center, so I just bring home those cheap (free for me!) black plastic pots. I wash them out thoroughly, cut a hole in the side, and sometimes glue a piece of plastic over the top since the pots can have holes in the bottom for drainage.

Gardening centers often give away the smallest 4 inch pots for free and sell larger ones very cheaply. We sell gallon pots for 10 cents apiece.