There's a website called Surplus Shed that someone else linked. I found 25mm diameter, 40mm focal length double convex lenses for $4.50 each. You can put those

There's a website called Surplus Shed that someone else linked. I found 25mm diameter, 40mm focal length double convex lenses for $4.50 each. You can put those parameters into the finder yourself here: http://www.surplusshed.com/lens.cfm http://www.surplusshed.com/lens.cfm