Please elucidate as to what optics you need.
6" is more the region of reflector telescopes rather than refractors.
If you really need to do a refractor objective, you can get the glass at Newport Glass works as they have kits for the various sizes. You will probably have to adjust the curves on the glass to fit your scope tube tho which won't be a really difficult job.
I will note that a doublet will be 4 surfaces to make rather than 1 of the reflector but those surfaces can be a lot easier to do as you just need to make them spherical rather than the much more difficult parabola surface.
Newport Glass Works site also has an article on how to do a refractor which tells of the necessary tools that you need to make (you might spend $40 on the tools themselves) which aren't complex to build.