While my own research makes plain that the Cub-owning community seems to consider the idea of installing even the most basic autopilot into a Cub a mortal sin, there are scattered accounts on the web of J-3 owners operating with installed autopilots (mostly for long-distance cross country flying).
But as I tried to determine what such a modification would cost, I quickly discovered that autopilots are expressly, explicitly, and strictly the realm of STCs (emhpasis added):
D.13h - Autopilots (AP), Flight Guidance Systems, and automatic flight control systems (AFCS) or flight directors (FD) - STC
Meaning that nothing less than a pre-existing Supplemental Type Certificate would make such an installation legal. Since STCs are pursued by the aircraft manufacturers (or the 3rd party vendor who wants to open the market for their product), I presume the costs of such are prohibitive (millions of dollars).
Are the folks claiming to have such equipment making it up? If not, how did they get around the lack of any existing autopilot-related STC for the J3?