A m/cylinder from a Suzuki GS500 or Kawasaki Ninja 500 will work and it looks very similar. Just make sure it's from a naked bike and not a faired bike, the latter may not have a mirror mount.
Also make sure it is a 1/2" bore master cylinder, some (with double discs) will be 14mm. The stock Triumph part is 11mm bore.
98 Thunderbird, 03 Speed Master