Even if you could buy the housing and lens together, you would have to crack them back open to swap over the internals! I would sand and polish your current headlights and then either install Lamin-X covers or put a really good sealant on every 6 months.
Here's a link to the Lamin-X covers. I'm planning to put them on my M in a few months after I respray the front end and put on a clear bra.
https://www.lamin-x.com/precut-film-...ht-covers.html
If you don't go that route, I used Chemical Guys Jet Seal on my Miata's headlights and re-apply it every 6 months. No issues with yellowing after a year and some change.