For a sidecar for a vintage Triumph, check Bob's BMW which is up towards Baltimore (Columbia maybe?) as they do a lot of vintage BMW restorations; they will probably not have the actual sidecar but can put you in touch with some of the local Brit bike folks.
Also consider posting to the Brit Iron mailing list (you have to join first and instructions are here
http://www.brit-iron.org/) There's always eBay but those are hard to come by so you will need lots of patience if you go that route.
Have you considered one of the new Urals? IIRC you can get a bike and sidecar, brand spanking new, for under $9k. I've had many a vintage British bike and while I love them, they are fussy, high-maintenance, and you might enjoy a newer, less tempramental bike more.