Is there anything available that can be used to coat metal findings to keep them from tarnishing?
I've used a spray fixative on base-metal before. The fixative comes in a can like spray paint, is available in matte and gloss finish and can be purchased at your local hardware store. This type of finish can wear off depending on the item's use.
If you're referring to precious metal, I wouldn't recommend a fixative, but would suggest storing your metal findings in plastic zip-top bags until you're ready to use them, and then storing your finished pieces in organizers. There are also anti-tarnish strips available which you can put into your storage container or bags and prohibit tarnish from forming on your metal findings.