It depends what you plan to use it for Tony.
If you want something for the roadside to swap a wheel if you don't have run-flats a scissor-jack from an E46 works just fine and fits the Z4 jacking points. It also doesn't take up much space. I bought one from Quarry Motors for £20 because I carry a full-size spare in my car.
I bought a low profile trolley jack from SGS Engineering for use at home - I think it was less than £40 delivered! I got a jacking pad to fit the BMW jacking points on Ebay for less than a fiver.