I was using 1 quart paint containers that I sell at my store but that was becomming expensive and they are a bit large for seedlings. Not to mention they are transparent, which people say are not good once the roots start to get exposed.
You can usually get free pots from nurseries that have to pay to dispose of or recycle them, or just buy cheap pots. I always recommend square pots since they hold more soil in the same amount of space and they fit nicely into nursey trays.