At least in Kentucky both Kroger and Target have them randomly in the imports section of the cookie aisle if you look for them. (Depending on supply chain presumably.) In Kroger it's often the hard to see/hard to find top shelf. They rarely get the fun or the especially good flavors, but they often have the originals.
I don't think Kentucky has an especially large Australian influence, so I assume that they distribute Tim Tams somewhat broadly nationally, but I don't know.
