If you are looking for big pots, check with some local landscapers - they put the plants in then have to get rid of the pots and around here they are charged a premium to take them to the landfill since they don't decompose. Usually they will be glad to save the cost of hauling and the tipping fee at the dump.
I have been given a whole 4X8 trailer full a couple of years ago just by asking and he was really glad to get rid of them.