There is 3W motorsport in Wintertaur, they sell some motocross gear.
http://www.3w-motosport.ch/
Also if you go just over the border into Singen there is a Heine Gerike store and another motorcycle shop opposite (between WalMart and BurgerKing) that only sell clothes (its a while since i have been there so i dont know how much is motocross).
http://www.hein-gericke.de
In the city I highly recommend Ernes for anything bike related, they do have a clothing in the basement but it is mostly KTM and Triumph branded.