Gonna post 3 links for ya dealing with this topic… and reading through them, you’ll get a sense of what’s involved (or at least, what WAS involved) in availability & installation if you do find them.
Bottom line… they were made by Lippert, and the policy used to be that there were not sold as aftermarket units. In other words… if your rig didn’t come with them, you couldn’t get them. But I’m sure that’s because they were a relatively new & unique option back then… but as they’re sorta commonplace now, the policy has probably changed.
Anyway, the first link kinda gives some history as to what I just said:
This next link is much more detailed. It shows options for a “homemade set” using parts from a Home Depot… and starting with post #41, also details my installation experience from a set of OEM Lippert 3-Season Doors that I happened to stumble across on Craigslist.
Finally, the Trekwood RV parts website does show them as being available… but the price is staggering. Here’s a link to the webpage, and a screenshot of it: