I need a simple 20-25mm diameter (slightly larger can work if it's plastic) lens with a 40-50mm focal length. It's for a lightmeter I'm building. I'm just having a hard time finding anywhere online or in the real world where I can buy such a simple lens element.
How about lens/viewfinder from a disposable camera or a box camera, it is a bit smaller bit might work. If a complete lens block is ok for you then those 40/2.8 objectives/viewfinder on cheap 70s compact might work too.
Try an optician that makes their own spectacles. Some of them have a little 'factory' on the premises. See if they will let you have a lens blank. These are about 40cm diameter. You'll need a 20 dioptre convex lens with no prism or cylinder.
Suplusshed had exactly what I was looking for. I was able to buy several 20mm diameter, 50mm focal length DCX lenses and a 20mm diameter, 17 mm focal length, for $4 each.