Climax Photo is stocking polypropylene negative sleeves in sizes they used to only make on a custom basis. They also, of course, carry common sizes like 4x5, 5x7, and 8x10.

The larger sizes are:

4x10
5x12
6.5x8.5
7x11
7x17
8x20
10x12
12x20

I use their 4x10 sleeves and have been very happy with them. They ship to anywhere in the world.

For more info— https://www.climaxphoto.net/wide_format_sleeves.htm