Browse SoundCloud for hip-hop instrumentals. Many hip-hop producers, both up-and-coming and established, release their beats for free on this website. You can listen to streaming audio of any music on the site, and much of it is downloadable in high-quality MP3 format. If the beat that you want to use isn't downloadable, send a message to the musician asking if you can use it; chances are, he'll be happy to send you an MP3.
Download hip-hop instrumentals from CCMixter.org. The CCMixter community shares samples, remixes and tracks released under a Creative Commons license. This website hosts hundreds of searchable hip-hop samples and beats, all of which are free and legal to use. If you use someone's beat on a track, you can then upload it back to CCMixter to share it with the community and get feedback on your work.
Use instrumental beats from the Freesound Project at Freesound.org. Unlike the other sites, Freesound focuses on shorter samples rather than full-length tracks. This allows you greater control over the way you use the beats: You can download hip-hop beats from the site's archive, then loop them to whatever length and at whatever tempo you need. Most of the beats on Freesound are in high-quality WAV format, and all of them are completely free to use.