I'm assuming you're a teacher, as I once was. My most successful runs at free posters were from the consulate general, the Spanish tourist office in the US and your local travel agent.

In addition, I asked wherever I went in Spain for any poster I could see. My rationale was shoot first ask questions later. I would go to any store and ask for anything. (I got a huge novelty-sized ATM card at a bank just because I asked...and that was in the US!) People are more than willing to part with worthless stuff when they feel they're doing something noble like helping a teacher.

I've got some other "fuentes" of free stuff, but I need to research from my list to get that info to you. Look back here in a few days.