Various RV suppliers handle these lens, maybe CW, almost any RV Parts place can get them. Purchase some 3M 08897 Dry Silcone Lubricant, put a little on a rag, wipe down the mounting part of the lens, it will never stick again.
While you are at it, change the hot halogen light bulbs to LED's and save ton of electricity and heat. Various places sell the bulbs, but Kustom Coach Works, Junction City, OR sold me mine, and gave me great price. Call them at 541-501-3371. Tell them Monty recommended them, he can get you the lens as well.
Monty & Janet - 2007 Alpine APEX 40 MDTS
S/N - 75715 - Retired - Master Certified RV Tech