Missing The Mylar Insert For Littleton Album

Hmmmmmm. Have you tried ebay or a Google search for them?

I don't think that the plastic slide material is Mylar.
I am reasonably sure that it is made out of acrylic, which is just as equally good for coins anyway.

Why go to all of the problem of trying to buy a slide when it is perfectly easy to solve the problem for free?
Clear hard acrylic sheet is used for food, toy and tool packaging. If it is used for food it is OK for coins as well.
The clear plastic slide you need can be recovered from this sort of packaging and cut to size to fit your coin album.

I am not all that keen on coin albums that have a slide over the coins, especially if the coins are in high MS condition. If there is any dust between the coin and the slide, the coin could be easily hairlined.
Littleton albums OK for building sets of coins in less than MS condition. Better than press-in sort of albums.
Probably helps to explain why some prefer high value coins to be slabbed.

I have for a number of reasons a bunch of extra slides for various Dansco albums. They are the same material as the Littleton ones. Send me or list here the size you need and I'll see if I have one that size or close.