Unfortunately S-TEC are the only ones that offer retrofit autopilots. The best hope for the future is the Avidyne DFC90 which pilots seem to like very much but as Peter said, they're not making much progress. So far it's only Cirrus and I believe now Malibus. Cessna 182 are "announced". Not even the new Extra 500 is supported which ships with an Avidyne Entegra 9 glas cockpit but a horrible S-TEC 55x.
My dealer showed me the internals of the servo and how they can easily fail in a way you get trim run away which is a great way to get killed!