You can file, grind, and sand the brass. The highest grit paper gets you a higher polish and I suppose you could even use some of the different rubbing compounds for a super shiny finish. As for buying, you can find dimensional stuff as well as pre-started guards at several knife making suppliers. A good place I like to check is the recycling places that also sell scrap. Have one down the road that I can dig through their brass and copper bins and buy it by the pound if I find some good stuff.
Jeremy