Wet custard shift sounds like the bushes in the linkage are all shot so that's where you want to poke, while you're there a short shift is nice and the way forwards is to post in the Genuine Parts section and zimmerbimmer will give you a quote for a new z3 shifter with bushes (definitely the way to go) which is not too expensive at all. Alternatively he'll probably be along here quite soon.
If you do feel like throwing money at it, I have the UUC motorwerks one,
here, and it's very nice indeed. Realistically there's no point throwing silly money at it as the z3 shifter is good and no point skimping on ebay tat as they aren't much cheaper than the real thing.