Best source for containers for seedlings?

[huntoften]

The past few years, I've been using opaque plastic cups that I drill a hole in the bottom. This works well, but the round shape doesn't allow me to put as many plants in an area as square ones would. They also tend to tip over once the seedlings get larger, but that's not a major issue.

I'd like to purchase a few thousand of the square 3 or 4 inch pots that commercial growers use, but not sure where best to find them. I can find them by the case, but the price is 2-3x that of the plastic cups I've been buying from Costco.

Anyone have any sources out there for cheap pots like these? Thanks!