Well...
I've been the eBay route, nothing. I have the service manual but it does not list specs on the belt, ie diameter and such. I tried an authorized Teac repair center and they told me the C-3 was a VERY old deck and they would have to get back to me, but I not holding my breath until they do. No I'm not the "Jingle Bell Rock" Bobby Helms.
...there are ways 'round everything...If you have the old belt, you could measure it yourself using a ruler that measures to sixty-fourths of an inch...more accurate would calipers, digital would be nice...if it has a circular cross-section a micrometer would be ideal...Insofar as belt length, any old ruler will suffice, I'd deduct a minimal amount for the stretch factor...too loose and it might slip, too tight and it may put too much load on the various bearings...
As I mentioned, any decent electronics parts shop should have a selection...bring the old one in, they will probably be able to measure it for you.
Re: TEAC/Tascam...I meant the mfr. themselves, not an "authorized" service center...there is little if any incentive to embark on what may prove to be a fruitless (non-profitable) search from their POV...
jimHJJ(...ain't life grand?...)