I came across the Beams seatbelt website. They recently created a website to sell directly to the public. They are the same belts that the Triumph suppliers sell, but half the price! Plus, they offer a broader selection of colors, lengths, and styles than the TR suppliers. They are sold with or without mounting hardware kits. They can also replace worn out webbing and rebuild seatbelt retractors.
I opted for the classic lift latch style with a 12" sleeve so that the buckle doesn't flop down onto the seat.
http://www.seatbeltplanet.com/TR6Canuck