Go to your local nurseries and ask, they'll sometimes have used pot bins (some charge). Ebay can have some good used deals on tree pots, I've used them a couple of times. The best deals for a small-time hobbyist I've found is going through the online greenhouse stores. The legalization movement has fostered quite a few of those, and you can find very decent deals ($1-$3 per pot) if you look around and take advantage of holiday/coupon deals and buy enough to qualify for free shipping.
NOTE: there are two types of plastic pot production. Off the top of my head I want to say one type is stamped and the other is blow/injection-molded. Not enough time at the moment to refresh my memory, but one is better for durability/longevity, IIRC.