Some Walmarts will carry them, but you will have a greater chance of success by buying your style of clearance lights at any truck stop.
...You can also try sealing your old lens with your favorite epoxy on the plastic and silicon where it meets the backing plate to get by.
You will have a male and female end to unplug and re-plug. It's a bit precarious if I remember correctly, but easy to do. The key point is that you need a TALL LADDER. I.e., you don't want a standard ladder or you might risk a trip to the emergency room with a broken arm from your fall. So don't risk it!
2004 Itasca Horizon ISK-40AD, 350HP Cummins 8.3L
115K miles and the best of 3 Diesels I have owned thus far!