POSTED UP HANDBILLS: Do they have to be particularly post-apocalyptic at 28mm scale? You can barely see them.
I swept through a few image searches and found such fun as a wrestling poster for the thundercats vs he-man and the masters of the universe, invitations to join GI-Joe's COBRA organization, weapons and anatomical charts, punk rock promo posters, hot girls with guns pics, and more such nonsense. I made a word file of a ton of them scaled down to appx 15-20mm size on the long side, printed them out, and are still cutting off ones for terrain pieces to this day.
At first glance your players won't notice, but eventually somebody will take a closer look and it'll be HEY.....this is....! and you'll get a laugh. If you really want Fallout related stuff you'll have to specify that in your search, and Fallout is very self-genre driven.
Now for full-on billboards, you'll have to either look further, buy some, or design some in Photoshop, with or without the assistance of an AI program to get the art style right if you want something quick and easy.
Have fun getting a sheet together!