The SAM4s SER-6540 uses 44mm bond paper rolls and the ERC-18 ribbon cartridge. The 44mm bond (impact for dot matrix) paper and the ERC-18 ink ribbons were once the most common cash register supplies on the market. If you have the SER-6540II the paper and ink required is the same.
We recommend you always keep paper and ink in your SER-6540II even if not issuing receipts. Operating without paper and ribbons in the register can damage the printer and you don't want to do that especially now that parts are hard to get. We still offer programming support, and limited parts like keyboard covers, cash drawer inserts and more.