http://turntablelab.com/index1.html
That should be the Ultimate Breaks and Beats section of turntablelab. For original tracks you should go digging but if you're trying to recreate or just kind of practice like we discussed in another thread in this forum, you can find a lot of the samples used in classic hip hop songs.
Edit: Go to funk soul disco jazz then when you're there scroll down to UBB and Alpha Omega.