Nicholas Smith and Mr. Muffin appear to be pretty large volume of both Lionel/MTH products combined as well. They, along with the ones mentioned above, are the main guys I look for when my LHS doesn't have what I need.
MTH might surprise a lot of people by still being around when Mike Wolf is kicking back with a beer in 1 hand and his DCS in the other and his retirement account smiling with him a year from now. I am told by my LHS his employees have been pretty much running the company, anyway, for the past two years . I'm doubling down on MTH, as i continue to buy their stuff. Their products are too nice to disappear completely.