I bought a length of orange plastic pipe at a B and Q Warehouse for around £3. It's fine for our 2 Netherland dwarves, but probably not big enough for larger bunnies. It was a lot cheaper than buying a tunnel from a pet shop though, and it can stay outside in the run.
I am pretty tight so I prefer to make or adapt other stuff into toys. Yesterday I bought a water butt base from Homebase for 99p (reduced from £14). On it's side it can be a little tunnel, but I am going to cut a hole in it so it can be a hidey house for them.
They also have a wicker basket that was £2.50 at Asda. I bought a few and just used one for the rabbits. I went back to get more for the house but they had put them up to £7!! Really wish I hadn't cut a hole in one...