Depending on your area finding a general contractor or handyman with experience with greenhouses would be your best bet. I would try to visit some greenhouses they have already built to see if the client is happy with the end result. More than half of the greenhouses I have seen were poorly designed (or not at all) and did not meet the client's goals for the project.
Hiring someone with greenhouse experience to do the design for the structure can be very advantageous, if there are no contractors with experience in your area. The greenhouse expert can design the greenhouse based on your desires and climate and then hand over building plans to a contractor for the build.