I got this one from a few other LJ authors whose work I regularly read. Basically, I find memes when someone I know does one. I would suggest looking around for original writing communities, though. There are quite a few on LJ, usually based around a genre, and the administrators often post writing prompts, questions, or suggestions to help out. You can always try Googling that sort of thing too, as there will probably be a few helpful sites that specialize in memes.