I agree with choosing very minimal criteria and trying every horse that meets them...like 14-16hh, 7-15 years old, and $xxxx and that's it. Don't try to replace your old horse, and try everything. I found my horse by word of mouth, and I don't know if I would have looked twice at him in an ad. He was primarily Western trained (former ranch horse), not much to look at, especially in his winter coat, but I went to look because he fit my minimum criteria...and I found an absolute gem. He was beautifully responsive to the aids, quiet, and willing. And cute. Really, really cute. He had no dressage training and didn't know how to use his back-his go-to resistance when he's nervous is to hollow out big time. He's not fancy enough for rated dressage or hunter shows, but he cleans up locally in hunter pleasure and I've won Western pattern classes on him as well and he's gotten some respectable dressage scores. He has made me look at horse shopping differently because if I'd seen an ad for him, I probably wouldn't have bothered...what a mistake that would have been!
Do you ride with a trainer? Ask him or her to ask around their connections and see what's available, and go look at everything, no matter where it's advertised or what it looks like in the photos (or hen you get there as long as it's sound). Ask any horsey friends to keep an ear out as well...sometimes the best horses aren't advertised.
ETA: 15 would not be a deal breaker for me if everything else was as you had hoped. With proper care, you could easily have ten years or more with a horse that's 15 for what you want to do.