Labadie's Bakery, Lewiston ME
This is the place known for their whoopie pies. It's in a weird neighborhood, but well worth finding. When we walked in, there were literally thousands of whoopie pies, most marked for their destinations around Maine, such as Casco. These were in little plastic bags, but you can also get them fresh from the case, along with donuts that looked excellent and other old timey pastries such as bismarks.
Finally, perhaps because of the low overhead, the prices are great. Small coffee plus pastry special...$1.75! And the whoopie pies are quite large too.
The "Lobsta Truck" in Los Angeles California gets their whoopie pies all the way from Labadie's in Maine, and charges a whopping 4 bucks per pie. I guess no price is too high for those Angelenos anxious to get their hands on our east coast treats!
We have at least one food truck here in DC that specializes in lobster rolls. I'm sure they are tasty...but I don't feel the need to pay $15 for what is probably a standard-size (as in small) roll. Also, $3.50 for a whoopie pie, but at least they are baked locally.