Ed Raly and Joe Williams (The gun Works) make them up to 2 bore.
I have used them both in the past for 8s 4s and 2 bores and have been impressed with their quality. Joe can't make them very long, but Ed can, so I have used more from Ed Raly, but both make good barrels.
Whey you say you want to try a big bore, do you mean you ant to try building one, or shooting/owning one, or both?
I have made quite a few in 8 bore, and a few in 4 and one in 2. So if you need pointers, let me know.