Yeah the 3/4" hex was the one that was suitable for drill motors with a threaded 3/8" mounting hole. The best option is to make your own now, with the most common method being to melt a 3/8" UNF nut into a plain bore plastic wheel. I did this by sitting the nut on an 80W soldering iron for a minute or so then pressing it into the plastic. Bit of a tricky process to get the nut both concentric and straight so that your wheel doesn't wobble but usually good enough for the job. Not the strongest method as it can wear down over time causing the wheel to come off, but should do the job for a few events at least.

Either plain bore Banebots wheels (with the bore drilled out to 12mm) or the blue wheels from Bearing Boys are the favoured options available at the moment, but anything with no bigger than a 12mm bore should work. I found that if the bore was bigger than 12mm, there wouldn't be enough plastic to melt around the nut, resulting in a weaker connection.