Any shop that sells salomon bindings should have them in stock, however they may not want to sell you just the plastic part, you may have to buy a new brake. Go in and chat to them, chances are they have some old brakes that they may be happy to spilt apart and just sell you the part you need.
Just looking again, if its the metal part that holds the brake onto the binding, again try a shop, but you may have less luck. Salomon offer it as a spare part, so you could get your local shop to order you one. There are pretty cheap too