Ah, yes...a proper distinction that I should have made from the get-go. I've not had the Rocky Mountain variety, nor am I curious.
I know the distance is a problem, but I'm hoping that as Denver's restaurant scene improves, perhaps fresh seafood (not just sushi) will improve as well...
Assuming you mean the ones from the ocean, not Rocky Mountain.
I have had good oysters at Jax in downtown denver (there is a location in Boulder to).
Also McCormicks accross the street from Jax. Part of a chain but usually good fish. They have a good happy hour menu which I think includes oysters.
Also Ocean in Cherry Crek. Did not have oysters, but the fish I had was great.
Others might have better suggestions. Of course "good" is somewhat relative for any fresh seafood 1,500 miles from the nearest ocean.