want to fly to Maine and drive down the east coast to look at lighthouses. Is there a route already established?

The New York times recently had an article about a lighthouse tour of New Jersey -- I think it was last Sunday's Travel section.

The National Park Service has a guide to the lighthouses on Cape Cod.

The AAA map of Cape Cod has lighthouses marked. Some are on private land and require a bit of creativity to view.

While driving along Long Island, this URL will be invaluable:

www.longislandlighthouses.com

If you do an Internet search for

"state that interests you" +lighthouses

you'll generally find a URL similar to the one I note, with descriptions, photos, and notes about LH's along that state's coast.

I found this great list
http://lighthouse.cc/menu.html
pick a lighthouse to see a photo and whether you can visit it. There's over 60 lighthouses on Maine's coast. Several years ago we took a fascinating cruise sponsored by the Maritime Museum in Bath to see lighthouses. The narration was great and we had a picnic lunch on a little island with a lighthouse. Sorry, I couldn't find any current info to see if this cruise is still offered.
On this website there is an itinerary for the most prominent along Rt 1. Since the beginning is right over the state line in Kittery, you might find better airfare to Manchester NH which is only about an hour from Kittery. If you decide to fly into MHT, mapquest will tell you to take Rt 101 east but it is much more scenic to go north on Rt 93 to Concord NH and then Rt 4 to Portsmouth. Rt 101 is boring limited access hwy. The Rt 4 approach will take you to Durham and a beautiful drive over Great Bay (I think you switch to Rt 16 ... have taken this route for over 30 years and don't pay attention to rt signs). Of course maybe you want to start at the northernmost point and work your way south.

http://www.scroope.net/lighthouses/lhlist.htm