I have C&C cubes, which I originally used when my first bun lived inside, before we bonded him and he moved outside. I think they're fab as they're very easy to put together and you can create quite a large space, divided up in interesting ways, with different levels, hidey holes, etc. The one thing I would say is it's probably best to reinforce the structure with wooden rods or something like that as, otherwise, they can be quite flimsy once they get to a decent size. Also, make sure you factor in enough squares to put a "roof" on as it's easy to underestimate how high a bun can jump!
I got mine from eBay, I think.