Its all hit and miss. There was a guy in Walmart yesterday, and he literally had 25 to 30 of the Matchbox cars in his cart, multiple copies of the same ones, pretty much cleaned out a lot of them. I was glad I got the Nova on Tuesday, would have been bummed out if I missed it.
I don't know, maybe he's got a lot of kids and nieces and nephews, the point being you just find what you find on any given day.
I don't make special trips just for diecast, but if I'm in the store I'll generally take a quick look.