actually you cant even change fks. or at least shops cant because to get at the brakes you have to take apart mechanical components of the brake which legally the shop isnt allowed to do. If somebody really really wants a wider brake on an fks or pivot then it should be sent in to rossi/look.
all other brakes can be swapped our pretty easily with one or two quick screws.
First find out the width of your skis. After you know that type in "Dynastar Brakes". You want to get brakes JUST wide enough for your skis. If the bindings are too wide they will drag when you carve, if they are too small they won't deploy (aka you loose your skis).
If you can't find brakes wide enough try and get as close as possible. My brakes were 130mm but my ski was 133mm, the guys at my ski shop just bended them. It's pretty common to bend your brakes.