I was just beginning to research this myself so thanks for posting! It would be nice to enjoy the lower cost and extra heat capacity of pure butane when the temperature allows. You can refill a used mixed-gas canister with butane but you'll still be carrying a 3-4 ounce canister that would probably weigh less than an ounce if it were designed for pure butane.
Building such a Franken-Canister might make a fun, challenging welding project for someone who knows how to weld. I'll bet no one sells them because some joker would try to fill one with mixed gas.