OT: rubber drive belts

I seem to collect more bits of vintage hi-fi equipment than I do computers these days (they're much easier to find around here)... Anyway, I've got an early-'70s receiver with a built-in 8-track deck where its drive belt has turned to black goo in the same way that QIC drive pinch rollers often seem to do (and oh how that stuff gets everywhere). Exact part replacements by manufacturer / model are unsurprisingly unavailable - but I expect that the belt material is the same composition as that used for e.g. floppy drive spindle belts (or at the very least, floppy drive belt material would work nicely). So... I could measure the pulley diameters and spacing, but does anyone know of somewhere that sells such belts by length/width rather than "this is a belt for xyz"? (and, if such a place even exists, do I need to correct for belt stretch, or is that factored into the sizing already?)