The main slip printers I use are the Addmaster 7100 and 7200... they're the same printer but use different cartridges. You can find them cheap on ebay, or at least you could. They speak their own little dialect that as far as I know isn't supported by cups as shipped. Even then, cups wouldn't be a good choice since there's a little more back-and-forth than just blasting out data to the printer. Same goes for other printers that support ESC/POS... it's not entirely the same as ESC/P and when you get into slip/validation printing you have to do a little more work on the software side.