From the photograph in post #3 it looks like an SMPS and the proximity to the power switch and what looks to be a filter inductor and capacitors I would guess it's across the AC input lines. If I'm right, and it works on 100V (strange AC mains voltage!) it should be rated at around 150V.
As a rule of thumb, whatever the AC supply voltage is, multiply it by 1.414 to find the peak voltage expected across it then add 10% to get the correct voltage rating. You can use almost any available TVS with that rating.
Brian.