I don't know where you can get it... but Home Depot sells mesh (~$3) for an 6" X 3 foot section. This:
https://www.homedepot.com/p/Amerimax-Home-Products-36-in-x-6-in-Hinged-Unpainted-Gutter-Guard-85280BX/100002884
I've used it on the front grills and the side intakes. Fabricating a couple of pieces for my diffusers is on the calendar... but only penciled in. I'll get to it. Shaping it to those diffusers without scratching up the part is the challenge. That, and securing it to the parts without looking all wonky.
Now... if you find the custom mesh please report back.
Cheers!