You don't need one unless you get your lenses very, very dirty. Breathing on the glass and polishing with a clean tissue or cloth will get it clean 99% of the time. Generic isopropyl alcohol for the rest.
Doesn't breathing on the lens risk introducing moisture, which in turn will cause rust, mould and the end of civilisation as we know it...Breathing on the glass and polishing with a clean tissue or cloth will get it clean 99% of the time.