I've bought abrasives of all kinds from this company off & on for some 35 years & was shopping them today for widebelts (for a "timesaver").
Assuming you will buy a range of belts, not just one at a time?
As can be seen, they have all the common type of abrasives in most size belts.
FWIW, my RAMCO/Tannewitz widebelt sander takes 25" x 72" belts which are a complete oddball - most sanders either take short 60" belts, or longer 75". My price for custom length belts in the past has always been smack dab between those listed catalog prices. so they are (or were) not gouging on custom lengths.
I might or might not make this current order with them; for my app, there are some interesting other options online, but I have never been dissatisfied with Red Hill Supergrit, other than the time i placed a rush order on Friday with CC#, and around the following Tuesday or Wednesday they called me and asked if i wanted to go ahead and place the order?.