An alternative to buy lenses is to make them. I just made a navigation light for a 1/72 Spitfire out of a piece of clear styrene sheet. I cut a little rectangle of clear that was a little larger than the nav light. I then cut a short length of sprue, filed one end down so that it's cross section matched the plan view of the nav light. Then I super glued the piece of clear to my sprue, so I now could hold it while shaping. Using my Flexi-file, and fine sanding sticks, I quickly shaped the clear to match the nav light. I then took a fingernail polishing stick which has three progressively finer surfaces to it. Although extremely clear at this point, I gave it a coat of Future for good measure. Now I cut it of the end of the sprue. To clean up the bottom surface, I but it on a fine sanding stick and carefully scrubbed it around with the tip if my finger, again repeating the process using the fingernail polishing stick. Lastly a little bit of silver paint on the underside, and it was done.
Although it sounds like a lot of work, I really only spent maybe twenty to thirty minutes on it. I used the same technique to make a headlight lens for a 1/35 T-34 tank.