Hi piston head. I got the ball joint pack from EDS,
http://cgi.ebay.co.uk/330462019865 . It cost me around €72 with shipping, which I think is a fair price for 6 joints.
Some people here in Lithuania use some Opel lower ball joints (apparently, from Vectra A/ Astra F). That requires adapting the lower wishbone design for the Opel joint, however apparently fits into the Sierra upright without modification:
