AnimeJapan2020 Cingeki Trading Badge This one is kind of embarrassing... They sold these badges in blindbags, sets of 5. I ordered 5, thinking they would send a box with one of each. Nope-- I got five random badges, and no Hiromi. I tried to order again, but forgot the deadline and placed the order a day too late. I haven't seen this set show up secondhand at all, either... but I won't give up!
Minna no Sekichan The original, from 2013. It's the only one I haven't been able to find for resale. According to the organizer's website, they were still selling the original for 300 yen in late 2015 (when the final Minna no Sekichan was released) so it wasn't a super-limited thing... If I wasn't easily embarrassed, I'd just ask Nova-san if there are still any copies left.
Cingeki Cafe Tickets I know of at least two other Cafe tickets that Hiromi has appeared on. I don't really seem them around anymore, but because Hiromi isn't the focus in either of them (and is actually just a speck in one) I'm not too concerned. ... but I'd still like them.
Any Fangoods If you happen to see Hiromi in an Artist Alley, featured in a zine, or on anything that can be bought, please let me know! It's harder to keep on top of these sorts of releases, so I appreciate the heads up.