Toy stores are good places to look for plastic balls.  I remember some lightweight hollow plastic balls like they use in the ball pits for kids, most were colored, but I have seen a few white ones. Dollar stores have cheap toy balls.

Puppetainer, do you happen to know the diameter of these "eggs"? I figured that for the size the head is (I completed that today) I would need 3 1/6".
I'm going to post some pictures later, if anyone's interested in the progress...

Okay Fabe. Only for a fellow puppet maker would I do this at 2 in the morning. Yeah, whom I kidding? I'm up this late all the time. Anyway the baseballs are pretty much exactly 2" in diameter which is going to be smaller than what you're looking for. It seems to me that hey did have bigger ones there but I don't think I bought any because I didn't expect to need them quite that large. Happy hunting!

Thanks. Yeah, that's too small. Still might be good to get a couple for other projects. Maybe they have bigger ones as well. Thanks again.

Re Styrofoam. There is a "foam" usually with the styrofoam but it is much denser and does not squish.  I cannot think of what it is called. I have some upstairs, will check it when I go up there.

I found those Christmas decoration balls at Michael's. However, the girl there told me that they discontinued them so she had to look in the back. I don't know if she meant discontinued forever or just for now... They're perfect for my puppet, so I'm happy.

Probably just because X-mas is a bit out of season now...