Where to find a belt for my REVOX B77 MKII reel 2 reel?
Your question is a bit unclear, do you need to know where to find a new belt (title) or where to place the new belt (text) or both ;-)

Anyway, i have here a (swiss) german service manual for a REVOX A77, perhaps some informations from there can help you. The manual is some 30+ years old. The visible parts of your photo correspond good to the graphics in the manual.

The (german?) order number for the A77 belt is: '31. 99.0106'

Under the right bobbin board you should find something that i would translate as breaking roller. According to the graphics there should be a pulley integrated in this roller. This should be the place to attach the other end of the belt.