I have every Loona poster, so I'll comment what I can on their rarity.

Before yyxy, the solo posters are quite rare. The 1/3 posters are also fairly rare with OEC posters only being a bit less rare. I would say for before yyxy, only a few thousand of each solo exists with a bit more for the subunits. Heejin's probably has less than 1000 posters.

Heejin's of course is the rarest and may only have about a 1000 or less posters. Hyunjin and Haseul ones are fairly rare, but Yeojin ones are really rare.

Vivi ones aren't too rare, while Kim Lip + Jinsoul ones are quite rare. Choerry ones are rather common oddly enough.

Starting with yyxy, the posters become much more common and were quite easy to find in 2018 and 2019. You could get them for free practically. yyxy subunit ones are very common as well.

After debut, the posters became very common and easy to find. Probably the only tough one to find is XX Limited A, but even that isn't too hard to find.

Heejin's posters are the absolute rarest followed by Kim Lip and Yeojin's poster sets. After that its Hyunjin and Haseul along with 1/3. After that its Jinsoul's and OEC, with Choerry being somewhat rare after that. The rest were relatively easy to find, but in 2021 are becoming problematic.