You will find pocket rhyme dictionary or you can always google rhyme dictionary websites just write a word there and you will find dozens of rhymes of that word, another thing I have been rapping since last 8 years so my advice is with regards to The Game interview "To be a good musician you need to be a good listener first" so I recommend you to start listening some good rap music strictly RAP not RnB etc e.g Jay-z his rap quality is awesome, Eminem, Dre, The Game and the best is 2pac so that would help a lot as well as watch rap battles on YouTube will help you understanding the pitch and flow. * hope this will help**
Most freestyle rappers use pre-written rhymes in their battles. It's very rare that you see a freestyle rapper make something up on the spot (ex., makes fun of his opponent's clothes, stance, look, etc...). I'd say study a rhyme dictionary, but if you really wanna be good, get creative and practice rhyming about everything you see. Practice on your friends/family, as well.