If you just literally need a no-frills sleeve meant to go in another bag to protect it from other things in the bag, this is what I got:
https://www.aliexpress.com/item/Top...1-12-13-14-15-inch-Women-Men/32907597149.html
Just keep in mind it is literally zero frills. No pockets, pouches, zippers, buckles, trim, padding, pen clips, wire access holes... nothing but a simple sleeve. They didn't even bother to put the stitches on the inside.