I got some in Walmart a few weeks ago. It was in the toy section (of course), in the isle with the really cheap stuff. I found it just laying on the bottom, wasn't even on a hanger. It wasn't name brand Silly Putty either, but a cheap China knock-off.
Just last week, I was in the local dollar store and saw it in the toy section there too. Same exact stuff.
There's only just a little bit more than a marble size amount in each egg BTW, so you might want to grab a few packages.