As far as the OP problem goes, if you measure the size, you can probably find a window. It’s most certainly not Lance specific. I’d look at parts warehouses that sell parts for any RV.
Kayteg1, you say you have newer cars, so you probably don’t need any parts for a 30-40 year old Mercedes. ;).
I don’t believe you really think a one off product like a Lance model RV that litterally had at most hundreds made should keep parts as long as Mercedes that makes tens of thousands. Anyway, it sounds like you are complaining that you could buy a Lance part at Walmart and are upset that you didn’t pay Lance to keep one on hand. That’s an awful problem. :D