Nice looking sofa! I've never seen sheets designated for sleeper sofas. What does Crate and Barrel recommend? They might make sheets for their sleeper sofas. I don't think even the older mattresses were 4". Perhaps you can get elastic clips to hold the fitted sheet in place?

LOL, I do remember. However it's even hard to find sheets that aren't sold in sets. Crate and Barrel doesn't make any sleeper sheets. I did find some online, but they don't appear to be available in store (I like to feel), or they get bad reviews.

Do you sew? It's very easy to make fitted sheets. Lots of online tutorials.

Walmart and similar usually have open stock sheets though if you don't want to sew.

Or you could approach an Etsy craftsperson and see if they could do custom work for you.

The Company Store sells nice quality open stock sheets/pillowcases.

I do sew and have made lots of crib sheets. Depending on how they're made though, it can be a bear to rip out all that elastic. Most is serged on. Thanks for the recommendation on the Company Store.