Use a rap or hip-hop dictionary to look up common rhymes. These dictionaries are made specifically for rappers to help them when they are struggling with words and need some suggestions. Websites like Dillfrog, RhymeZone and Rhymer are easy to search and offer long lists of rhymes.
Listen your favorite rap songs. Jot down some of the best rhymes you hear, and create a working list for yourself that you can either commit to memory or use to lay down a track. Pay attention to nontraditional rhymes like slant rhymes, which are imperfect rhymes like "man" and "when" or rhymes that only work because the two words have similar suffixes — "prosperous" and "glorious," for example. Understanding how to use slant rhymes effectively will give you many more options.
Read the liner notes of your favorite rap CDs or look up the lyrics to your favorite songs online. Some people have better luck committing things to memory when they can take something apart visually instead of just listening to it.
Listen to other genres of music that emphasize songwriting and rhymes. Using the same rhymes and terms that all the other rappers are using won't help you stand out from the crowd. Examining rhyming patterns in other forms of music might spark some new connections and prevent you from getting stuck in a rut.