There are many to choose from, the cheapest are probably the M-Audio Oxygen keyboards. If you're not sure how many keys you're going to want (25, 49, 61,76 or 88) then think about the kind of parts you're going to be wanting to program, if you're wanting to play in keyboard/piano parts for example with a large range in pitch then a controller with 61/76/88 keys is going to suit you best. If you're most interested in programming beats, arpeggios, lead lines etc then a smaller keyboard will probably suit you best.
More expensive boards such as the M-Audio Axiom, Emu Xboard and CME UF keyboards feature sliders and rotary knobs to assign to virtually anything you can edit in reason (or whatever software you're using), these can be handy as it's never a bad thing to have as many different control options as possible, also be wary that they can take up more space, the CME and Axiom keyboards suffer from this in particular.