I have a TW bents (elegance, 2 wheeler). I suspect it uses the same type of idler.
I didn't commute today, so I can't go look, but the last time I had it tuned up, Ajo bikes replaced one idler, probably with a T-cycle. I'll see if I can find a part number.
You could probably just measure the width and diameter, and call T-cycle or one of their local vendors, if you have one, too.