I've heard egg yolks mentioned as the richest source of it. Not sure how much, if any, is in the whites. But its a pretty inexpensive supplement.

They did a rat-maze memory test and the rats getting extra choline did better on the maze. The real interesting part is they were middle-aged rats. And baby rats whose moms consumed lots of choline had larger dendrites with better connections (or something like that).

So this middle-aged rat figures her memory can use all the help it can get!