Have you looked through the range of Downhill tyres. They are big and wide and must come in hard compounds. I doubt they would come in white, and also they are usually covered in brand names. But maybe these could be removed somehow. Sanding them off.
Or what about if we can find a hard compound tyre and remove the brands names or repaint them white. I remember seeing a thread on tyre paints. Cant recall what the results were.....
Im sort of hoping you will find the answer before im ready to buy more tyre. I just bought another cheapy whiteys for my Villiers build. Ill see how long they last then i will need an alternative also.
Or finally is there a nice, decently priced white (or otherwise) motorcycle tyre out there? And run 26" motorcycle rims..... but are they available cheaply?????
So many questions.... lol