Best place to buy is direct from FN/Browning parts. There have been several threads in the past which included specific part numbers and the the phone numbers. If you don't want to call FN/Browning the next best thing is Midwest Gun Works, there prices may be slightly higher than going direct but it's not bad. The resellers on GB usually charge significant more than buying direct. Why pay $140 for a F2000 flash hider on GB that is $79 from MGW or direct from FN?
The guy on GB does sell some parts you can't get elsewhere..... Buy at your own risk. There is a reason legitimate dealers don't make/sell things like auto sears and timed hammers for converting the FS hammer packs. You can buy into his BS that it's somehow legal but don't be surprised when the ATF comes calling. You wouldn't buy an unregistered DIAS for an AR-15 from some guy on GB would you? As Admiral Ackbar said..... It's a trap! Or maybe just entrapment.