If you deck is valuable and you want a very high quality replacement, then go to
www.turntableneedles.com
I always purchase deck and turntable belts from them, bit pricey but the quality is as good as the original. Speed accuracy is also great, and the investment is worth it, especially if you have a high end deck or a nakamichi deck.
I have purchased belts from the local markets too, for budget repairs on the mass market type decks. They tend to have the following problems:
1) rubber is sometimes a little too thick and they tend to harden over period of time (much faster than high quality rubber belts)
2) they tend to perish faster, especially if used in humid climates, cracks form and they just end up breaking
3) when they start perishing, the rubber residue or deposits that are hard to clean (and leave stains) especially if they dry up and harden on the flywheel
4) after some use, they stretch marginally causing pitch issues
price wise, turntableneedles.com is expensive and they are apparently charging shipping as well due to the pandemic situation (shipping costs have gone up). Picking up a belt from your friendly neighborhood electronics spares show would cost you 20 bucks max