year one, or classic industries, or a host of other suppliers. I know year-one had a chart in their older catalouges to help you deterimine which speedo. gear to buy, depending on gear ratio, tire size, etc.
Look through the for sale ads here. I see them for sale quite often. In fact I have a few myself. Some I want to keep but let me know what one you need and I'll see if I have it to spare. Price will be a bunch cheaper than new.