Punch in the cartridge number or Lexmark or whatever in the search function on eBay. I bought 3 color plus one black ink Official sealed HP cartridges for my HP Deskjet 970 for $100 (incl shipping). I think I saved between $30 and $40 over Staples/Costco prices.

I emailed 4 or 5 dealers on ebay with the exact cartridge and asked a price for 1 black and 1 color. Upon getting prices I made a counter offer.