You don't buy, you make one - total cost less than a few bucks *if* you have to buy anything at all. I used a piece of rebar, battery and wire to de-rust an old motorcycle tank. The same concept works to defoul and decopper a rifle barrel (except you use a much finer steel rod).
How do you like the 7x57? I have a Ruger #1 in the same chamber. Easy on the shoulder and not so easy on the other end. Great cartridge.
la plus belle des ruses du diable est de vous persuader qu'il n'existe pas!