If I were looking to hire someone to do real plaster work with lime putty based plaster, I'd talk to the people that oversee the historically significant buildings in my city. They would know who not only does real plaster repairs, but whether or not they're any good at it.
Another option would be to phone the places listed under Drywall/Plastering Supplies in your yellow pages phone directory. Find out if they sell both dehydrated lime and Plaster of Paris. If they do, they would also know who they sell it to, and whoever is buying a lot of those products is doing a lot of real plaster work in your area.