The SAM4s SER-6540II uses 44mm bond paper and an ERC-18 ribbon cartridge. Both the 44mm bond (impact) paper and the ERC-18 ink ribbons were some of the most common cash register supplies when these were current models. If you have the SER-6540 without the -II as far as the supplies go they are exactly the same.
You should always keep paper and ink in your SER-6540II even if you don't issue a receipt. Failure to keep paper and ribbons in the register can damage your printer and you don't want to do that especially now that parts are scarce. We can still offer some programming support, and limited parts like keyboard covers, cash drawer inserts and more.