Rebirth is a (now discontinued) TB303/808 emulator from Propellerheads. It is now FREE to download, but only works in OS 9/Classic mode.
Find it here to DL free:
I bought it when it first came out and loved it to death. Nice to see its out there for free tho!
You will need the Apple Loops utility if you want your loops to be GarageBand "friendly". You can get it here:
You take a loop you have created, open it into the utility and save it out. It marks the beats and makes the loops change within GarageBand to speed/tempo changes etc. Very nice to make your own loops more functional in GB.
I use it convert all my ReBirth loops, samples, etc.
I found that most of the best hiphop loops are from familiar older songs. I esp like some of the drums from jimi hendrix, deep purple, led zep for that sort of thing.
Your other choice is to find a loop you like the pattern to but not the drum sounds and then rebuild it using drums you do like. its pretty simple in GB. I take a loop i like, place it into a channel. Then create a new channel and find kick, snare, highhats, etc i do like and put them over the old beats corresponding items. Then turn the old loop to mute and play it looping. If it sounds off, scoot the pieces over, repeat. When you got it how you like it, export it to a sound, convert to an Apple Loop, and import and bam.
also has some nice loops, many available in sample packs to test out (maybe you can find something in the free samples oyu like)