It should be available in any hardware store and is fairly inexpensive (or it was). You put a few white crystals in an ounce of water and mix it up. Mine looks like salt so a pinch of salt is what I use. I use a small stiff brush and dab it on. It will work in less than a minute in my experience. It will bleach stains especially the black ones where metal meets wood. It does not leave a stain. I rinse the wood off in cool water since it is an acid and you probably don't want to leave it on for long.

So many guns, so little time!