The side of the road.
Particularly after spring planting, my neighborhood and the surrounding area has tons of pots out for streetside brush/bulk trash collection. They range from the cheapest, flimsiest plastic pots to some of the highest quality plastic pots you can get, as well as the occasional terra cotta or ceramic (I tend to get those at yard and estate sales). Most of these pots are relatively small, but I do tend to get two or three a year that are 30-40 gallons.