Any of the race car shops, Ed Quay, S&W, ART, Alston's, Quarter Max, etc., etc., can sell you the tubing and the bending tool to make the window frame but you'll have to weld the tabs on the tube and then to your door. Personally I don't like Lexan as it scratches easily unless you buy the expensive stuff (MarGuard) and the glare off it makes it difficult to see at night, especially if you are older like me.
If you don't have fiberglass doors than the Lexan is a very, very small weight savings and to not have the convience of roll down windows just makes it worse, to me. My car still has all factory glass in it and the front windows still roll up and down.