In 2001, I purchased a Hubodometer from Abbot Industries. It has worked perfectly, came calibrated to my tires, was priced right (I think it came to $24 with shipping) and came to me in 4 days. I mounted it onto the hub cover for my Bambi.
It's put on over 30k miles and still looks great- no corrosion or rust, no moisture, and does what it was advertised to do.
When ordered, they select the unit based on your tire's revolutions-per-mile, a spec that is available from your tire manufacturer's website.
I would go there again with no hesitation if I needed another: Abbott Enterprises - hubodometer