check out the DJ Target's Aim High mixtapes (they're actually CDs not tapes!), volume 1 isn't so good, but volume 2 and 3 are fantastic, volume 2 being the best grime mixtape i've heard. to be honest there aren't that many female MCs, there are a couple whose names i can't remember, but the reason i can't remember their names is because they don't do very much.
anyway, the best way to listen to grime is to listen to the pirate radio stuff, so tune into rinse.fm (they have webstreams) they play grim in the evenings most days. also look at these websites to download old radio sets and mixes
http://www.barefiles.com/
http://oldgrimemixtapes.blogspot.com/
http://hosted.filefront.com/adamantiummusic