For the few of us that don't incessantly bounce 'twixt here and TMP, there was a conversation resurrected from 2010 on the plastic sheets.
http://theminiaturespage.com/boards/msg.mv?id=186572As I said there, we colonials seem to have plenty in 7-count and 10-count, but 14-count is a bit rare. However, the difference between 7- and 10- is sufficient to get by.
Alas, the best place I found locally as far as variety selection was HobbyLobby, and I'm boycotting them. JoAnn's comes in second. As I've said elsewhere, first trips can be intimidating to the male, but as long as you learn to filter out all the pastel colors, the folks can actually be quite friendly.
I did find a large pile of squares, with a couple of 10-, at a thrift store, for pennies per sheet, so have plenty for the foreseeable future...
Doug