Try eBay, I just found this for £61.95 for a full regulator and motor..
http://www.ebay.co.uk/itm/2005-BMW-Z4-Passenger-Side-Front-Window-Regulator-Motor-2-Door-Cabriolet-E85-/162044685250?hash=item25ba9dabc2:g:XVcAAOSwKfVXF69E
This link is for the passenger side but I'm sure you'll find something similar for the drivers side.
When I did mine I bought the cable replacement for £18 and did the job in about 2 hours, it will depend on what condition the motor and cable loom is in. If the plastic cable spool is broken you'll need the full kit.