I have a few sources...
Most often, I draw about 2-3 random lines with no idea of where I'm going, and after about the 4th or 5th line, I've got a direction going...(usually turns into some sort of eye or simple, cartoon-y character). This plays heavily on boredom, and is really good practice for
Dadaism, for which I oh so love. It's basically where you let the art take you, instead of you taking your art....if anybody remembers my phase a while ago where I made like 1351235 shitty 100x100...they were almost all entirely done in this fashion.
When I'm trying to improve my creativity, I bite from the people who get their ideas from heavy drug influence. Seriously... I'm with Kon....let other people suffer the damaging effects. There's this kid in my grade, and I swear, he's the next MC Escher (been trying really hard to get him to pixel)... and a lot of my "tripped-out" stuff is influenced from him....
Another good substitute for drugs is dreams

. I haven't pixelled much based on dreams, but I think it's a great "untapped market", and I think, correct me if I'm wrong, isn't that where Dali got most of his ideas? (though looking though galleries of his earlier stuff, I can see blatant paraphernalia references).
I often do something similar to the 100 images thing, too....search google images for a while and you'll be sure to find something worth while to attempt.
And then, for the most extreme cases of artist's block: do a still life.
But mostly for me, it's a dada thing. Whereever the pixels take me, I follow.