You could try http://www.northfarmstud.com/rehoming.php they advertise in H&H. There is also Doncaster sales & there is another website that is dedicated to ex race horese but I cant find it at the moment, I actually deleted the link last night!!
depending on the purpose of searching for an ex racer www.exracersclub.com is an online community that offers support to the ex race horse owner and anyone that has an interest in them. The club is run by and organised by ex race horse owners that can offer support and advice and have first hand experience of ex racer.
Their aims are to promote the ex race horse and help both horse and owner in their new careers.
As the others have said. But please do not think that an ex-racehorse is a cheap way of getting an eventer! I have now 'done' 3 of them, first was a decent straight forward mare who was reasonably successful at BE Novice and is now in foal, second one was quiet and relaible but did have a bit of a stubborn streak and he was rehomed and is continuing his education and my boy Sovereign who has finally turned a corner with his competition nerves and is now starting to fulfill his promise.
Sovereign was given to me for free, but over the last 3 years in schooling,hiring venues, paying for hc restarts etc he has easily cost me more than any straightforward 'off the peg' horse. That said I wouldnt swap him for the world, but do be ready with plenty of time, patience and money!
The cheaper ones at the sales quite often have less than perfect conformation so bear that in mind too.