One that's already been done before, but still a classic:
*Picture: Cthulhu standing at a pay phone, word balloon from the phone saying "The party will not accept the charges", word balloon from Cthulhu, "Nuts"*
Caption for above: Collect Call of Cthulhu.
Others:
Failed my Sanity check years ago.
I go, you go, we all go, for Mi-Go!
Why settle for the lesser evil? Cthulhu for President!
(I stole the last one, but darn if it wasn't funny!)
Gaming conventions are your best bet. I got my stuff years ago when The Armory was still doing business through the front door of their warehouse in Baltimore.
There's also a Pokethulhu t-shirt that I have. It shows cute little Pokethulhu swooping down with the caption 'Gonna catch you all'. Try explaining that one to the grocery store bagger. I've also got a Miskatonic University beer mug with 'Go Pods!' on it. Got those at wizard's attic, along with the aforementioned 'Collect Call of Cthulhu', and 'Cthulhu for President' shirts. It's my goal to get a shirt of every day of the week, so I never have to be Cthulhuless.