Buy them on ebay. Vancouver Mining supply stores will have them, they are the principle tool of a gold miner. a good one is $100, a cheap one will drive your eyes nuts. for gun work 10x or 10 power is lots, 20x is for biologists who study fungus and molds 30x for stamp collectors. Depth of field gets very small the higher the power, which is why 10 is good for guns as you typically want to see the imperfections in context of the surrounding area which is hard to do with a 20x and impossible with a 30x.