Seen as your from the UK too figured we could relate, not many people on here from UK! I want some rocks/drift wood too, any idea?
I went the rather stupid expensive route and got mine from pets at home. All I can really think to suggest is to find a small creek or stream and see what you can find there. No guarantees, or there's eBay. I wish I'd looked there first, lots of mopani (or whatever it's called!) on there for reasonably good prices. If I think of anything else, I'll be sure to let you know! Good luck, and yeah, not many on here from UK so it's good to find a fellow Brit!