Here is a bottle I found totally by chance in a small DC liquor store (shout out to Central Liquors on E St in Chinatown! Great selection), after having reading about it here and there. After trying - and loving - the Big Peat Cask Strength offering from the same company, I had to give this a go.
Apparently Rock Oyster had two small batch Cask Strength releases (this is the first of the two), but then was rebranded to "Rock Island," so you may see it near you under that name. Unsure if they have released any cask strength Rock Island.