Also, Walgreens and Big Lots. They HAVE had them. I got a couple of the silver xB2s at the local Big Lots. They are one of the "cheaper" cars at both of those stores. I've yet to see one at a Walmart yet, though the Walmart ones are the same manufacturer line.
Also, Family Dollar has Marvel character 2-packs and the Spider-Man/Green Goblin set has a red/blue xB2 as the Spider-Man one. I got s couple for $5/ea.