Try this:
http://www.guidedogs.org.uk/index.php?id=1720
they say there is a waiting list, you also have to be aware that the dogs have failed for a reason. It can be health related (in which case you will have lots of vet bills) or behaviour related. Also it means the dog will probably be over a year old and used to working.
Why not find a really good breeder - one that does all the health tests for the breed (hip score etc) and breeds for temperament and buy a normal pup?