http://www.andymark.com/searchresults.asp?cat=215
That (should be) the link to the "hubs" section of the andymark.biz site. You're going to want to find the size that coincides with the size of your output shaft/key. I wasn't actually around when we got them in and put them on, but unless I'm mistaken, you slide it over the output shaft, and the outer holes correspond to the (long) bolts on the inside of the wheels. There might be another part necessary to keep it on the output shaft, but that would be a good starting point.
(Someone correct me if I'm wrong, I had already left for college when they got everything together.)