Hey John, there aren't any off-the-shelf solutions that screw directly onto 3/8" drill shafts anymore. Robo Challenge used to do wheels that had nut inserts in them but they no longer make them. Most solutions are custom, the simplest of which is along the same lines as the Robo Challenge wheels, heating a nut and pushing it into a plastic wheel hub. Takes practice to get right and can often have a bit of wobble.

What size wheels are you after? I've got some Banebots ones with a hex hole through them that fits onto a corresponding brake pipe fitting with a 3/8" UNF thread. They're 98mm (104ish with bike tyre fitted), not sure if that's any use. Made a video a while back talking through that method: