I cannot recommend a specific forum for TR6, in my experience I use many sources for parts and advice. I live on the east coast and usually purchase from:
The Roadster Factory—Triumph and MG Car Resoration Parts
A good source for info:
Buckeye Triumphs -- Home Page
Vintage Triumph Register has a lot of info:
Triumph Car Club Vintage Triumph Register
I often use eBay for parts.
Lastly, look for auto shows in your area and chat with the owners.