The gallery paper, Halloween paper, museum paper, post-office paper, and snowman paper are rare unorderable stationery. I think they are DLC (or maybe people can only get them by hacking, I don't exactly know). I have seen people selling them here on TBT, so they are a real thing.
No, they're not hacks, villagers (RARELY) send them to you. I had some museum paper given to me by Norippa in my Japanese game. They're insanely rare to get that way, but that's the only legit way I know how to get them without dupers giving out copies.
I find it really disappointing that you aren't able to order those unique types of stationery, if it is true that villagers will mail it to you. I would use the Halloween stationery for everything if it was reorderable and I don't see much of a point in locking it off. If any of my villagers ever mail one of the rare stationeries sometime, I'd probably construct a sign next to their house like "#1 Villager!" or "Villager of the Year!"
My two side players have received the museum and post office paper.
At the time those 2 side players weren't writing letters. Read that they send the paper to get you to write back.
Yeah, they sent me paper with a hint that I'm supposed to use it to write them back with it- like not doing it will really do anything! I used to write them everyday until I got that badge, then was like forget it, too time consuming, and wouldn't you know it, a letter appeared with that attached, and it's like yeah, sure I'm going to use paper that I can't reorder for THAT!