How wide do you want it? In M42, best deals might be for 28mm focal distance.
I would recommend the Takumar 28mm 3.5 - excellent lens, plentiful and not expensive.
Many older no-name brand 28mm in M42 are quite decent lenses if not opened too much. I have an M42 Focal, which is not bad at all. Tokinas, or East German types should also be good enough.
Yes Scott, I am referring to the screw on type. Back when I had a door peep hole affair that worked well, but I took it apart to clean and destroyed it. I used it on Oly D-490 Zoom.
Now I am looking to buy one that screws into the filter threads of my 18-55mm lens.
Ron, the only wide angle auxiliary adapter lens that I have any knowledge of is the marexar UltraWider. I have one. It is designed for use with a 50mm lens on a 35mm film camera, giving a focal length of about 28mm. If I attach it to the front of my Nikkon 18-55mm lens, it only works at the long end. If I zoom to around 35mm it vignettes badly. I think that all wide angle auxiliary converter lenses work this way. You may, however, find a fish-eye adapter that will work, but the image quality is likely to be very poor.
GrahamS
Age brings wisdom....or age shows up alone. You never know.