You can purchase the lens at any authorized Ford dealer like Palmetto Ford truck, Metro Ford Inc and Midway Ford. You can also buy it online from lamin-x.com, autoanything.com, nextag.com, familycar.com, innerauto.com and amazon.com.
Dealerships tend to have the highest prices. Check with an auto parts store (Advance Auto Parts, O'Reilly, etc.). If they can't order you one they will know who can. I got two complete new headlight assemblies that way for about half of what the dealership wanted to charge me for one.