It's an extremely common part! You can order it on
Amazon, (Bargman 30-80-21), and have it delivered to your doorstep!
It's also available in amber, if you have moths or insects issues.
And it's, (the shipping actually), is ridiculously expensive, I hope that you are sitting down!
If you have PRIME, it's actually cheaper to purchase the whole fixture, just to get the lens!
http://www.amazon.com/Bargman-30-80-...TNDPGN9JB11AWM