In that case (and I KNOW they are swamped) but the FBI or at least some enterprising US journalists might want to canvass 100 or so random Army-Navy surplus stores in the USA, near population centers near Arab populations (New Jersey, Michigan, etc), to see if there were any 'bulk' purchases recently.
Just one route to go down; might not lead to anything.
I am sure the FBI is already looking into any source for the uniforms. They have known about this for awhile.