Window unit AC size question

I am interested in installing a window unit in my CT thru the wall. As thing go with time, it will need to be replaced eventually. I don't want to cut a hole in my CT to fit one AC unit and not be able to find a replacement the same size. Has anyone considered this or have found this to be a problem. I do have a roof top AC off my old 5th wheel that works great. Had plans to use it but in my overthinking process I considered the cost of replacing it when it goes out
and the most cost effective route I can come up with is installing a window unit instead. Which takes me back to the question about size consistency of the units. I'm starting to feel like a dog chasing his tail while beating a dead horse! Out of all the Southern Engineers that are on this forum, I can't be the only one that has considered this.
Thanks
Earnest

Thanks
Earnest