Last week I got about 300 pots and a load of trays from work all for free from where they were planting up all the planters and beds. They were all the good quality sturdy ones as well.
We've asked people with skips before, and had all sorts from carpet, to wood scraps, wheels, pots, trays etc
We even found a fly-tipped push lawnmower once which was really good. - we didn't have to ask for that!
Damon