Not sure where you are in the UK but there is this on ebay at the moment, collection only.
http://www.ebay.co.uk/itm/1415333892...%3AMEBIDX%3AIT
Also Plastics4Performance will manufacture a Lexan replacement door window for the Z3 but as they are for racing they are a bit pricey (circa £90 each).
http://www.plastics4performance.com/...or-window.html