Ebay. I have a HP printer and use compatiables. Seem to do the job just fine. Sometimes you can get bargains on there from "one-off" sellers. Brand new HP XL black off ebay saved me quite a bit.
When printing, don't forget to use draft if printing text.
Hi gina, I get mine from PriceStorm they sell originals and compatibles. They seem to be fairly cheap compared to some and only half the price of PC World and WH Smith. Here is a link to their site it's worth a look.