Do you want a
1. MIDI controller keyboard that does not have it's own sounds, but will play the sounds of instruments within Garageband?
2.a MIDI Keyboard/Synthesizer - with it's own built-in sounds?
If it's 1), then a few brands to check out:
depending on what other knobs, faders, etc you need or whether you want weighted keys check these out:
M-Audio Axiom 49
M-Audio Oxygen 49
Akai MPK 49 (MPK49)
Novation ReMOTE 49 SL Compact
Korg Kontrol 49
Novation X-station 49
I haven't used all the above - so do check them out in more detail.
Let us know if you have any more questions.