Training wheels for adults
This sounds like a really bad idea. Training wheels limit your ability to lean into a turn. If you're going fast enough to balance without the training wheels, you're going fast enough that you need to turn by leaning.

So you would need training wheels that retract when you get above ~5 mph, and deploy below that speed. Apparently something like this does exist for motorcycles, but I've never heard of anything like it for bicycles.

The alternative would be to ride a trike.