I know there are several smaller peonies that are generally used in rock gardens. I assume they have smaller, shallower roots because you would expect the soil to be shallower than in a traditional garden bed. Try a search on Merry Mayshine, that's a fernleaf hybrid that's small enough. Probably any of the reputable peony sellers will give you a list lf suitable candidates. I've had really good customer service from Adelmans Peony Paradise, they're very good about answering queries. Good luck!

Just looked at Klehm's Song Sparrow Farm website, they have a section for rock garden peonies. Their website is www.songsparrow.com, just look under the Plants tab and select Rock Garden Peonies, they have 15 plants listed.