This is an interesting topic for me, since I love rap music.

I looked into it a while ago, and as far as I can tell, there are probably around as many lesbian rappers as there are straight female rappers. I could certainly name more, off the top of my head.

Anyhoo, if you wanna check any of the out, google;
GOD DES + She [somewhat alternative rap, for some of their songs]
Mz Fontaine [she is, in her own words, gangsta ^_^]
Lady Luck

There are a few more I can't quite remember at the moment.