Most people I know who own them, get them off the internet. GHD themselves promote www.jasonshankey.co.uk who have them on for �89.99 + �1 p&p.
You could also try www.hqhair.com where they a bit cheaper.
I have never seen them in high street stores although I live in a smallish place and maybe the stores dont hold all that much stock.
Personally I own a GHD straightener but have used various other cheaper makes and have always had the same result??? I do think nowadays you are paying for the name and why is it the 3 salons I have used in the last 6 months or so dont use them!!! LOL