Hello,
I would like to replace some of the original inner sleeves that are old and falling apart.
What is recommended? Acid free paper? Plastic/vinyl?
Thanks,
David

There are already some great topics on this subject, you can search in the upper corner.

Always make sure there is plastic inside the paper.

Good news is that LDs don't matter value wise when using stuff like acid free so you are free to use whatever you can find.
Search up LP inner sleeves, I have when possible been using the antistatic elephant condom styles, I've just dealt with the issues they have and take my time.
These are U shaped and sometimes come from Japan, they are all plastic but sometimes called ricepaper.

You can find paper with lined sleeves and those are great but really best for USA discs, but these tend to tear on the edges as plastic is in a U shape from
front to back with nothing on the sides.

After talking about it for years I made a video about this subject, more or less. It’s in my signature.

useful Youtube post as I was thinking about the very subject the other day especially the MGM inner sleeves on their discs