My church is planning to have a plant sale in the spring. While I was at a Meijer Store I spotted a cart full of returned pots and plastic trays that were going to be recycled. I explained the spring plant sale to the rgarden employee and he told me to help myself to any of the stuff on the cart. Here is another source for pots and trays for seed starting. Just ask at the garden centers at any big box store and I'm sure they will let you take them.
HWB