My Mom was able to dig up a site called zazzle.com that has all sorts of Marvel Comic images. What's even better is that any one of these images can be slapped on a onesie. Go grandma! Here's an example. Saaweeeet.
So I've been googling for X-Men onesies (specifically Wolverine onsies--but who wants Michigan Wolverine stuff). This is all I can find so far (pretty lame):
Let me know if anyone finds any cool Wolverine onesies, though. Sticking Logan in one of those would be righteous!