Where to get a .4" sacrificial lens?

I have a mini 904nm diode and the lens is so small it can't easily be cleaned and a little dust messes up the beam.

In front of the diode is a .4" cylinder shape area I can put a sacrificial lens. I guess I can either screw a module into this place or bond a lens in this area.

Any idea where I can get a .4" diameter shaped glass lens or a .4" module with a glass lens that will not change the beam shape? I want it to be glass and not easily scratched.

Thanks

Any ideas?