For new (replica) spares for GPO 200, 300 and 700-series telephones, such as cases, cords, dial centres, finger plates etc., I can wholeheartedly recommend:
http://www.gpospares.co.uk/
The chap who runs it, a Mr Ray Sutcliffe, is an absolute gentleman, and the parts I ordered were of excellent quality and arrived almost instantly. Well worthwhile if you have a nice 706 or 746 which is let down by damaged or faded cosmetic parts, or even if you just need a colour-matched line cord with a "modern" 431A plug pre-attached.
When I ordered, I had to send an e-mail or phone, there's no online ordering facility
per se.
Nick.