Hi - there are six different versions in the Levy Collection of Sheet Music. Do a bibliographic search for Potomac, and the songs will come up. I gather the song is also known as "Picket Guard," but "Potomac" will find it. If you read the instructions exactly and push the correct (center) button, you can enter quiet Potomac and find just those 6 songs and not have to sort through others.
We have the lyrics and MIDI right here, if that'll do for ya. Just put Potomac in the search box on this page.
-Joe Offer-