Here's a web page at Quest Outdoor Fabrics that provides a couple of mesh options that the page says works for pack pockets (see the Leno mesh and Lycra mesh):
I don't know anything about this stuff. Found the above Quest link at the following thread on Thru Hiker discussing the same question you asked:
The Thru Hiker thread is kind of old (2007), but it provides links to possible sources as well as Quest that appear current and might have what you want.
One of those other sources, The Rainshed, has great photos of their mesh items and offer to send a swatch of selections for a dollar. See Net & Mesh Item #1278 at the The Rainshed website under "Fabric/Swatch Sets" --