Anywhere that sells belts. If motor factors cant, industrial power transmission etc. ( assuming PK section of belt which is typical size on automotive stuff )

10PK1219

Gates, Fenner, Optibelt are common brands.

Eriks in the UK do Fenner...no doubt there are suppliers all over Europe

https://www.fptgroup.com/Ribbed-Belts/

Hayley Bearings in the UK do Gates and possibly others.

Optibelt list 1200 and 1222mm belts. Damned if I can remember their main supplier

Most of the above should be able to source any width, as often they're just cut from a sleeve to whatever width the customer needs.

cheap unbranded listing.

https://www.ebay.co.uk/itm/10PK1200-...YAAOSwE8daYJTq

5PK1220 seems common......jury's out on running a pair side by side. You may get away with it, you may not.