First step, look around! Go to your favorite search engine and do a search on 'free rap beats' or 'download free hip hop instrumentals'. Look around and see what's out there and what's available. keep looking around -there are tons of them out there.
Here's a list below of some website i've found that offer free and royalty free rap instrumentals. I haven't checked them all out so use good judgment and read everything carefully before downloading anything.
www.flocabulary.com/instrumentals.html
www.openmindsentertainment.com
onebeatsource.com
www.soundclick.com
www.hiphopinstrumental.net
mobeats.com
trackbangas.com
www.hiphopandrapbeats.com/
www.rockitpro.com/category/urban
www.buffedbeats.com/rnbbeats.htm
www.underratedbeats.com/Beats/freebeats.htm
www.shadowville.com/index.php
www.freemusicdownloadwebsites.com/free-rap-beats.html
Next, when you find an instrumental file -I recommend going for .wav form rather than .mp3 format. While the size of the file may be bigger, the sound quality of the music file is much better in .wav format.
Now, check and make sure you understand the licensing of the file before downloading so that you know if you can use it on a demo tape or whether or not you can use it commercially. It's always important to follow the rules when using copyrighted materials.
Next, before downloading -take proper precautions against spyware and viruses when downloading your rap instrumental files. Protect your computer!
If you can't find the sound you are looking for, don't give up: keep looking around -or better yet produce your own beats. Making music isn't that difficult to do -it can be very time consuming, but there are many programs available to help you produce music. To learn some ways to make your own instrumentals in many different ways -feel free to check out some of my other articles. Thanks for reading and good luck!