I'm not sure I share your same perspective on what's rare. I haven't seen Ron get a shitload of his own art, he just makes appearances in group shots and gets shipping art with Hermione. Draco, Snape, Sirius, Remus, Tom Riddle and even Regulus Black are all way more popular with fangirls, and thus have a lot more art comparatively.
I don't have a ton of art of the real fringe characters. Often times they have pretty vague canon descriptions anyways, so fanart can be unrecognizable unless you're told specifically who it's supposed to be. I'll see what I can do, though.
Here's Dean Thomas and Seamus Finnigan drawn like they're lucky star characters. This artist has draw basically every character in the series, but always in this hyper moe style.