It seems apparent from the many advertisements in the 18th-century newspapers that most flints were imported. There are ads for many types offered for sale, and it is frequently mentioned that they are imported, just arrived, from Bristol, etc.:
French
best oil French
Dutch Gun Flints
English
"firkins of English gun flints , also best oil flints"
"a few Thousands of fine Gun Flints , which will be sold at 5 Shillings and 6 Pence by the Hundred"
oil gun flints
"black and French gun flints, agate ditto"
French oyl flints
best Dutch oil flints
common and best Oil Gun Flints
black and oil flints
There were apparently some people with flint-knapping skills, because at the beginning of the AWI Congress published this:
The Pennsylvania Gazette
July 10, 1776
PHILADELPHIA, July 10.
IN CONGRESS, July 4, 1776.
Resolved, That the Board of War be empowered to employ such a number of Persons as they shall find necessary to manufacture Flints for the Continent; and for this purpose to apply to the respective Assemblies, Conventions, and Councils or Committees of Safety of the United American States, or Committees of Inspection of the Counties and Towns thereto belonging, for the names and places of abode of persons skilled in the manufactory aforesaid, and of the places in their respective States where the best Flint Stones are to be obtained, with samples of the same. By order of Congress.
Here is a fascinating video of making English gun flints in the traditional way, for those who haven't seen it.
https://www.youtube.com/watch?v=7XPEoiY3NnI
Spence