Yeah, there really isn't much for lossless rap.
Even on the big trackers - What and Waffles - there is a lot of rap that isn't uploaded in FLAC - at least in comparison to other genres.
What Skizo said may explain it partially, but I still like to download the albums I really like all in FLAC for storage/backup purposes and selection has been pretty disappointing for me.

Does RaptheNet have a decent amout of FLAC? Or is it mostly MP3?

And by the way, someone mentioned Pedro's - Pedro's is a great lossless tracker, but it's hip-hop selection is kind of sad. They haven't had the category very long, and many people were hesitant to appreciate it. A lot of members have a "rap is crap" mentality and they're not too open minded about it. They have a few pages of albums upped, and they're good albums, but it's definitely not a good place to go strictly for hip-hop albums.