They're pallet collars. If you can find someone who uses a lot of pallets, you should be able to get a load for free as they tend to cost more to send back than the cost of purchase, so some places have loads going spare! We used to have loads at my old job, and I was going to do the same, but moved house and no longer need/want raised beds, so left them behind. They are usually heat treated as per pallets, so should last a few years.