Outdoor beverage vending carts and some of the CS restaurants. We saw the MM Snowman bottle topper/straw at the carts that sell bottled drinks. The gingerbread MM straw was also at vending carts, but at some CS as well. Basically you have to stop and look whenever passing a cart or CS venue. Usually they sold one or the other; not many had both. We were there the week after Thanksgiving and the gingerbread MM was easier to find.
We found both at Mortimer's in DCA. It's the first store you see on the right after entering DCA - the open air fruit and character bread loaf store near the lockers. We also bought some gingerbread MM straws at the vending stand next to the store opposite Soarin'.
Some mobile ordering menus included specialty souvies (e.g. the Mary Poppins spoon, ice cream containers) as add on options so you could check to see if any list the straws.