On belts: don't bother with the cheap ones on Amazon (30 belts for $5). Trust me, they are terrible. They are not uniform thickness so you end up with terrible wow.
I had good luck here:
https://www.ceitron.com/passive/belt.html
(Look up the store in Dayton, it even looks like a classic mom and pop electronics store).
One more hint: if you have a belt that slips off a pulley, most likely reason is that the pulley axes are not parallel with each other, and probably nothing to do with tension.