https://www.biedlers-belts.com/single-v-belts/polyflex-belts/ This is about the only place that I have been able to find the small V-Belts used to drive IBM and Smith Corona electric typewriters (among others). You can find most any belt you need on this site if you know its dimensions.

I was able to buy replacement toothed belts from an industrial seller via eBay by measurement. Actual Gates belts. Mine fit a Smith Corona electric 6TE and the part number is 3M315 but once you know the width and length it’s a matter of translating it into a Gates part number.