Beck (Indiana) and Vintage Motorcars (California) are both highly regarded around these parts, as is Intermeccanica out of Vancouver, BC in Canada. Their specs are quite different, so take a look at them, talk to the owners, and figure out a budget. Kitman motors is reputable if you want to build it yourself. I believe that they just sell bare bodies.
JPS has mixed reputation on this site, and Thunder Ranch has been out of business for a number of years unless they found a new owner. There is also a Vintage Speedsters in Arizona that I know little about.