Do they seem too tight when you put them on? Does the TT get hot inside for some reason, like it's sitting on top of an amp or receiver?
How long are these belts lasting? Days, weeks, months, ___ years? If you tape a broken one together, just long enough to see whether it seems to have stretched a lot, you might be able to know whether it would be accurate to measure one of them: length of the loop, and width. Then you could find a place with a good guarantee that has a good stockpile of old belts who would give you a better deal on any other belt that is the same size as your TT takes. If it lasts as long, or longer, and is cheaper (assuming it works just fine), then you've got a better deal.