When I was looking for my boy I went on gumtree, Facebook groups(I joined over 50!) horsedeals and horsezone. Hope this helps!
Horse Deals, or if you have Facebook there are lots of Facebook groups that you can join
Horsezone!! Also horsedeals is good too
If you want them for dressage there is a great barn that trains and sells horses, mainly thoroughbreds. It is located in Tamworth and called 18/24 Equine. I've been there and love the work with the horses. Go on their facebook page if you're interested!
Horsezone 👍 In my opinion