The best place is probably Douban, there are groups for rappers like Fat Shady and there’s also a 成都说唱 group with a bunch of posts and information. Looks like it hasn’t been active recently, but I think you’ll be able to find a lot of mp3’s on there. Another place worth checking out is 虾米 if you can access that from the states, there’s a lot of Chinese rap on there as well.