" Does anyone know where we might be able to find a shower door that is 78" wide that is not custom made and can be purchased for under $1500?"

Nope. Again like I have mentioned previously here is part of the issue of designing as you go with no selections made upfront. There are crap "Adjustable" units that may work but they are just that crap.

" Also, we heard that for frameless shower doors, it may not be as durable and would require more maintenance in the long run. Is this true? "

No.

" (Note: I don't think we can do frameless shower door anymore, as the contractor already put up the water barrier or water dam already (the black bar).. "

See my first and previous comment about not having a solid directions and ALL major things selected prior to beginning a project as opposed to flying by the seat of your pants.

" BTW, ChatGPT recommended these brands/models: "

ChatGPT has zero practical knowledge, those are pretty much all builder grade junk.

You need custom. And that will be more like $5500.

My SIL ran into this - she put up a shower curtain temporarily, saved for a gorgeous custom door and has enjoyed it since install

If you go onto Fergusonhome.com there are many shower doors that are 76 -78" wide. You would not select a door until the tiling is done of possible so you can get the exact dimension needed. They all seemed to be pivot doors and were all less than $2500 , some even less than $1000. Please pay attention to reviews, glass height and glass thickness.

If you wanted sliding doors, then your installers would have needed to add a small wall to one side or both, to close the opening down to 72" - not sure what stage of the process you are in as to whether that can be done or not.

Also - framed vs frameless is still the same maintenance IMO. Just make sure the unit you buy has a protective coating on the glass - it really helps.

Good luck!