You could also ask over in BF Benchmade forum, or go to BMC's forum n ask, or measure your washers OD, ID n thickness, n go to a knife parts store and match. I poked around and found a bunch of washers for specific BMC knives- grips, 710s, 940s, etc, but none explicitly for Freeks, though I'm betting pivot is not unique in size. You could email those sellers n ask for dimensions...
Folks have been known to swap washers of different OD and thickness sizes, so as long as ID is correct within reason (tolerances) it will work.