I was in Homebase this afternoon, and they have a policy of giving away cardboard boxes - they have a big stack of them past the tills, and a sign saying 'please help us to recycle - help yourself to boxes' or something like that. They also had a load of large plastic pot trays, the kind that they keep arrays of potted plants in on the outdoor shelves. I bagged a few good deep ones (that I reckon will be fine to plant seedlings/plantlets directly into), and also a flat shallow tray, good for placing pots on.
This was the Reading branch, don't know if others do the same.