I had seen them at Wal-Mart too, but the only ones I saw were in a two pack paired with a normal rider and bike.
That format (2 figures and 2 bikes) made the pack more expensive than I could justify, for basically just one skull head.
According to that link, the guy found them at Target, but as part of a pair again.
Why couldn't they have done them individually.

_________________
"Now I am somewhere I am not supposed to be,
and I can see things I never really should've seen,
and now I know why, and now I know why...
things aren't as pretty... on the inside"