Two manufacturers come to mind (at least in the U.S.): Hoodman and Extend-a-View magnifier. I tried the Extend-a-View magnifier today on my Canon A590IS/Kowa digiscope setup, and can't really say if I see any marked improvement in focusing. The instructions that came with the Viewer said to try "defocusing" the Viewer by rotating its eyepiece to minimize the pixel dots but I'm not sure if I see any improvement in the screen.
Ultimately I believe the most important factor is the focusing of the scope, and having good light.
Ultimately I believe the most important factor is the focusing of the scope, and having good light.