I've not seen aftermarket regulators for the Z4. Several members have replaced their own regulator complete with cables (part about £130 from BMW) but attempts at the cable didn't seem successful. I paid a trusted local garage who did it all-in for about £175 from memory.
A word of warning, from my own bitter experience frameless windows can be tricky to fit and adjust correctly, or worse still end up busting first time if they pinch
As you say the regulator is the likely culprit Part 1, the motor is part 2
http://www.realoem.com/bmw/showparts.do?model=BT52&mospid=47797&btnr=51_4859&hg=51&fg=10
