How many are there and who are they? And do they stock boats?

The only one I know of is Pier 57.

Correct Pier 57, and I don't know what you mean by stock boats because they are not mass produced, but yes they do have some there.

Jim Earle up in Port Severn is a dealer and always has a few at the Marina .
http://www.jimearlemarine.com/

Everything goes through Pier 57.

From what I understand Pier 57 has exclussive rights to the MTI line, I guess Pier 57 would have sub-delers per say and yes Jim Earle has sold a few to my buddies;

42 stand-up - 1075's
44 poker run - 1075
42 Pair-A-Dice - 1550 Sterlings
42 Stand-up - 1550 Sterlings