I got together everything I could find in the Vector Set Symbols thread here and from https://github.com/Investigamer/mtg-vectors. I had to modify a few of them to get them to display correctly in MA, but everything in the attached zip should be working ok now.

Outlaws of Thunder Junction Commander: Investigamer's Github repo has icons for this set, but the ones there don't scale down properly in MA. I'm not good enough with SVG to fix them, unfortunately.

Seems like no one has made the following yet:
Unsanctioned
Rakdos Guild Kit
Ponies: the Galloping

Additionally, there were a few existing ones in Sets.zip that had some issues:
COM19_M: this was shifted to the left a little in comparison to other rarities in the set. Adjusted the viewBox to be the same as other rarities.
KANEC_M, _R, _U: these were not displaying the rarity gradient on the left half of this icon in MA. Found replacements.
MOHR2_C: the file in the zip has a Cyrillic Es instead of a Latin C in the file name. It works fine after renaming it.