ashley
go to a bar and ask if they are throwing any of them out that you can have

AyyGirl
Spencers or Hot Topic have a lot of those kinds of poster and some neon signs as well. They are not free though. Going to a bar is a good idea or a liquor store to see if they have any posters to throw away. I doubt you'd find any signs for free though unless it was at a garage sale =)

spoony
The only free way would be to get them from someone who is disposing of them, but if you're willing to spend some money then most markets stalls would sell some or your average poster shop.

Grun
ebay. many sellers specialize in just that sort of thing.

Old E
im sure you can order them online, what kind of beer? Heineken.com, Newcastle.com

Ken P
google beer signs or beer posters and you will find a ton of places online that sell them.

BRAXATOR
The search took hours but you are well worth the effort.