Agree TracksUSA is the best place, unless you are willing to get a used one. Then search CL or eBay. Sometimes you can get a good deal on eBay on new ones too, but you have to know exactly what you are looking for.
Best part about TracksUSA is that they are just a small company, family owned and run right out of their old farmstead in MN. They do all the work, and treat the customers very well.