I bought one for mine from sears. They're in the lawncare section as something to put on your lawnmower. It's the exact same case as the ones you buy out of a catalog. It ran me $30 and when I had issues (stopped counting hours for some reason, but it is waterproof) I returned it and they gave me another one. The same thing from Parts Unlimited is $40.
You can get one that is also a tachometer for $43 that would be cool... but I don't know that I would actually get any benifit from it...
