That's a really good idea, I think the chaps at our recycling centre do stuff like that on the quiet, I once saw a woman going to dump a perfectly good guitar in one of the landfill skips (wood & old furniture), I've never seen the guys move so fast. I think they ebay the stuff and split the proceeds. So you could ask them to save you something suitable, or ask on Freecycle.
Aldi, Lidl, poundstretcher and Wilkos are all sources of cheap gardening things, if you have a JTF nearby that's also good (and you don't need a business card any more to get in). You could also try charity shops, they might have stuff that's suitable. You could probably get used tyres from a garage/car breakers for nothing, and use those for planting potatoes, just earth them up, adding tyres as you need to, and the beauty of that is there's no digging involved when you need to harvest them.