I helped Bradd (Hotbarrel) with the pistons and the original style muzzle brakes. I don't know what his current status is on parts, he may still have some in stock.
I believe PanaceaBeachBum makes compliance parts, as does ORF.
Of course OOW makes them however I don't know if they sell them separate of their "kit build kit". I don't know of others. Does Pirate make any?
I believe I got hte SGN article in their annual. Is it the one where they review the OOW rifle?