Don't know of online sources, but have you checked with your local nursery? I had no idea mine had them until I asked one day. This sweet old lady took me behind the shed, and there were HUGE piles of all different size pots, dirt cheap too. Had to give them a wash and a bleach as they were used, but they worked perfect on the cheap.
Hell you should contact commercial landscaping companies. I used to work for TruGreen landcare and we used to just throw away all the empty pots that plants came in. I would take them home by the truckload and distribute them to friends.
Yeah, a lot of them would just as soon have someone pick them up as have to pay to have them hauled away, especially in a city where they have to go to a special landfill.