Best Place to eat Pie
The Lancaster Dutch Market store in Gaithersburg has two shops selling great pies and cakes. (1) The bakery right inside the front door has excellent fruit pies and I just bought a peach praline and a mango/strawberry this morning. (2) Make a left and walk 30 feet to the cheese vendor who also has a wide array of cheesecakes, cakes (e.g. carrot), and pies (Key Lime, peanut butter, etc.)
I don't think you can eat your pie there (unless you visit the restaurant inside the store), but certainly you could eat inside your car or in the parking lot, or better yet just go home and eat your pies there.
Unfortunately they're open only from Thursday through Saturday, check their web site:
Especially this time of year local farmers markets are good places to find pies made with local produce. I'm seeing blueberry and cherry pies now and soon peach and apple will show up. In early fall, you can get pumpkin and sweet potato. The Alexandria market in Old Town on Saturday mornings in Market Square, the Upper King Street Market in Old Town, (by the metro) on Wednesdays 3-7pm, and Dupont Circle Market in DC all have bakery vendors that sell pies.