My builder mistakenly placed an LP gas fireplace in my house six years ago.
I only recently discovered the mistake and need to know who I should contact to get the best advice about correcting this problem. Do I contact a heating/air conditioning company, a plumber or some one else?

Depending on the brand, so are not convertible. You may end up having to buy a new one. There shouldbe a parts list and a service phone number in the back of your owners manual.

Agree with Josh. Contact fireplace manufacturer, or distributor. They will be able to tell you if you can convert from lp to ng. If you can, and you don't feel comfy doing it yourself, I would just start calling plumbing and heating companys to see if they do this style of work. Most heating and cooling companys will, and some plumbing companys will. If your unit is convertible, it usaully as simple as replacing an oriface or two, adjusting air shutters, and replacing a spring in the gas valve. What brand is your fireplace.