Amazon.com: Roland GA-FC Foot Controller for GA-112 and GA-212 : Musical Instruments
Boss GA-FC footswitch - Five Star Guitars
The GA-FC: Unlocking Performance Potential with Your Amp
Haven't seen a lot of examples of boards that implement the GA-FC, so here's the one I just put together! (Also posted in r/guitarpedals) : r/BossKatana