Do a Google image search for 'motivational' or 'demotivational' and you'll find a huge pile.
A lot of the 'demotivational' ones are older, a backlash to the clueless HR managers that truly believe that the original 'Motivational' posters improved the work experience. Then a couple of years ago the kids on 4chan got bored and picked 'em up, so you'll find a bunch for the younger crowd with themes on RPGs, lolcats and such.
And you can ALWAYS make 'em yourself. I use the generator at diy.despair.com, and there's another generator as well (I've forgotten the URL... age, y'know.)
We have a long 'forced' theme thread for motivationals over on Emily's site that LucyBoots started. >