Supply of original belts for these cars is effectively non-existant, but they are listed here for both historical significance and for the benefit of owners of cars still retaining original equipment who may require service parts.
Standard Triumph didn't list seat belts for TR2, TR3 or TR3A. They did, however, in the later TRs, make available fixings which would convert sidescreen TRs to accept seat belts. Working out where to attach seat belts (and which belts to use) in a vehicle not already equipped with mounting points is obviously not a task to be undertaken lightly; your life may at some point in the future depend on where and how you fitted the belts. We would strongly advise you to seek the advice of a specialist with plenty of experience in fitting belts to TRs before starting the job.
Also, if possible, get your seat belt fitment sorted out while the bodyshell is being restored, or at least before it is painted.
TR4s, TR4As, TR5s and TR250s have factory-designated seatbelt mounting points, and generally speaking all but some TR4s came out of the factory with seat belts fitted. Supply of suitable seat belts has been a problem down through the years, particularly for classic car owners who generally would prefer to fit something that didn't destroy the period ambience of their car's cockpit. Luckily the later TRs are nowadays blessed with a very suitable range of replacement belts, which while complying with modern regulations still look something like late 1960's/1970's products. They don't look exactly like the originals, however, but they will look the part, especially if both are replaced together.