I use www.supermediastore.com
Go to their Ink/Toner section.
Click on HP Inkjet (I assume you have an inkjet)
Then you can either look up the model of your Printer, OR, you can click the tab and look at ink by ink number.
When you look online, theres OEM (original equipment manufacturer) ink, which is made by HP themselves, and Compatible inks, which is the exact same ink but made by another company and thus cheaper
This site has really good deals every couple of weeks, like 25% off and free shipping.