The restrictions came AFTER some idiots monkeyed around with some units. The control part, aka the receiver is obviously not available directly to the public.
Try dealing with HK or SIG for spares. My money is on you not faring much better, and be ready to get a tongue lashing from the HK reps for being a customer.
An easy way to get spare parts is to just buy another complete rifle. If something breaks, you just rape the back-up rifle and call FN for a shipping label on the back-up. Simple. :?
PEACE THROUGH STRENGTH