Finding an instructor that has shown isn't that big of a deal to me. In fact, I've had incredibly bad luck with instructors around here that HAVE shown, as they seem to rush the horse through its training so it can SHOW, SHOW, SHOW. I do not have the money to show anyway, so quality training is key for me. I don't like shortcuts. I don't like force. I want the classical dressage where you were training a horse so that it'd stay with you during a battle, not high-tail it outta there!
Where the instructor learned their knowledge is important to me. My previous instructor went to Meredith Manor, then rode with the Royal Lipizzaner Stallions. Believe it or not, she did not tell many people about that. She didn't like to toot her own horn. Some of the other dressage riders at my barn looked down on her because she didn't show, didn't have top-notch warmbloods, and didn't rush things. She also didn't go to nearby clinics from "Gold Medal/Olympic worthy/etc." clinicians. She was a single mother trying to make ends meet. She didn't have the money to show, although she did have several horses that she had bred herself for movement and disposition.
Although some of her students showed, many of us did not. We just wanted a good relationship with the horse. Also, she worked extensively on building a good foundation, with the belief that "once a good foundation is built, everything higher up will come easier." Many of the people that showed regularly expected faster results, and they didn't get that with her, so they moved on to another instructor. Of course, these people have already bottomed out at 2nd level, because they didn't put the time and effort into establishing that foundation.
I knew my instructor was a good one. (Unfortunately she's moved now.) Every horse I brought to her acted ornery or had some little problem. After a few months of lessons with her, these same horses would enjoy riding so much more. It used to be that I'd go out to the pasture with a halter, and they'd all run. Now, I go out and they all come up as if saying, "Pick me! Pick me!" It's a good feeling.