When I was putting together signs and adverts for my 28mm urban board I ended up with a lot of bits and pieces. A lot of it was taken from a Deus Ex (the original) upgrade project, other bits gathered from all over the place. I tried to avoid having too many obvious jokes and references, though there are a few in there. But what I largely ended up with were:
Movie posters (fake and real), adverts for food, clothing, cybernetics, conferences, building companies (particularly on anything unfinished…), public health warnings (pirated chips (cybernetic control chips that is, not potato) can give you bipolar disorder) and I think coffee. Though of course the disclaimer here is that it was for an inner-city thing not a frontier town.