Polyvee belts and idlers

For measurements, the best way is to get a belt that's too long and then cut it down to fit, then measure it. You might be lucky but the string method can be quite far out, depending on the number of pulleys and whether the belt rides inside or outside each.

Google or yellow pages for an industrial belt supplier, they'll have a lot more sizes and are a fraction of the price of motor factors. 7pk is a strange one but you can very easily cut some ribs off an 8 or 10 pk using a stanley knife.

I design belt driven compressors as part of my job and trying various belts is the best way to get the right length, particularly if there's not a lot of adjustment and/or a lot of pulleys.

For idlers, I'd go to a scrappy with an idea of what you need and find something off the shelf. Or maybe you could find a motor factor with a parts book.