At Colonial Williamsburg, archaeologists working on the Anderson Armoury complex this spring came across a nice French button from the 53rd Regiment, which served as artillery during the war.
A button from the French 53rd Regiment, from here.
Button of the 85th Saintonge Regiment Infantrie found at Yorktown, from here.
Up in New York, the folks at Chrysalis Archaeology, a cultural resources firm, recently uncovered several Revolutionary War buttons in their excavations near the South Street Seaport. You can read more about the buttons here. Thanks to Joseph Privott for bringing these to my attention.
British military buttons recovered by Chrysalis Archaeology in New York City, from here. I'm away from my library and unsure which unit wore the "J 5" button at left.