I just returned from the Romantic Danube Viking River Cruise and there were several single cabins on board. Our next door neighbor utilized one of them and it was actually nicer than ours! She had a sliding glass door in her cabin and we only had a large window.
Check out the Viking River Cruise website to see which boats have singles available. I know they just built 2 new ships and I am sure they also have singles.
Good luck!