Heavy-Duty Stabilizer Wheels for Adult Bicycles, The Original Training Aid fo... | eBay
I agree with the trike suggestion. However they tend to cost a few times what a "normal" bike costs for the same grade of equipment/parts, just like tandems do too. The link I provided is for an "adult sized" training wheel version. I've sold and installed a few years back (when i still ran Bike One), mostly for riders with various challenges. They do work but are nowhere near as "nice riding" as a trike is, or stable either. One nice design difference with these stabilizers is that they don't attach to the rear axle, thus when used with a IGH or der system don't interfere with axle nut engagement on those short SA axles
Andy