You gotta use sounds that entertain the mind. Sometimes buying sounds just doesn't cut it, you gotta be a more creative beat maker.
Suggestion: Record real-life sounds on location, in the field. Create custom sounds and loops.
Get a recorder like the Tascam Dr-40 and go record some sounds on the spot. The bus station, railroad tracks, department stores, the mall, dark spooky alleys (you get the idea).
Then load the stuff you recorded into your audio editing program and make snippets of stuff you can add to your beats. Doing this sounds silly, but you will get some really dope results. This can serve as a tool to add atmosphere to your tracks.
Your beats sometimes need more than the sound fx that anybody else can get access to from sound design companies. Use this technique whenever you got nothing else to do and want to expand your custom sounds. This technique is not limited to rap beats and can work for any genre of music production.