There is no way it would ever happen, but I would look at my Instructors yard, as they have some very lovely horses for sale and also knowing me, would know what was suitable.
Got mine from Germany. I happen to be a friend of a couple whose father breeds dressage horses and he has some absolute stunners. I think the Germans do dressage better than us (controversial). He (the horse) can be a little horror at times but he is teaching me bucket loads.
I would go to the Show jump dealers/breeders and look for a showjumpers that won't make the grade jumping but has been out and about to a few jumps shows and has a good basic temperament and attitude. These horses go for reasonable money, will have been out and about and if it's a well bred warmblood should have no trouble getting to advanced, just not likely to be world beater.
I'd go to the Assoulines, as already mentioned, assuming I had a sensible budget. My old instructor trained with them, and she spoke very highly of them.
Diva came from Garry Williams - she was everything he said she would be. We have just come back from Emiles and he believes she could go Grand Prix providing we keep the hormones in check!
Germany is a good place but i beat you could find A really nice one here if you look in the right places. So look around on H&H But I would Go see Garry williams wont hurt to have a look.