Are you looking for a Lexmark toner cartridge? If you have the Model Lexmark X543, you may be running out of toner or already have the warning that you need to order the cartridges.
The cost ranges from $50 to over $100, newegg and buy.com both have them for the $50 range. You have to return the used cartridge.

Lexmark Return Magenta Toner Cartridge – C540A1MG