Here in Minneapolis the Dollar Trees have board, but you might have to ask if they have any in the back. Protesters are buying it up left and right for signage. A side note. A sheet of Elmer's foam board is about 227g while a sheet of Dollar Tree foam board is 114g.
FB back in stock here in Asheville, NC. You can save yourself a trip and actually call them up, and surprisingly, they answer the phone and know the status of FB. At least that's the case here, at 2 local stories. Way better service than I expected!