Does she want this 4/5 year old to be started and at an appropriate training level for upper level dressage plus good mover with likely high scores? That could be tough.
One suggestion I might have is asking around at breeders who are downsizing about broodmares in that age range who are well enough bred/good enough movers that they were bred young. I don’t think anyone who knows what they have will have a top youngster who is started and going 1st or 2nd level at least in schooling (depending on age/that individual’s philosophy) for that low a price.
Just my experience, but I found unstarted 2/3 year olds to be a much larger bargain - and less advertised because breeders know they’re worth more once started.