Gadgt is right...some of the older HPI wheels came with spacers in the 12mm hex shape to place inside the wheel as a spacer. Some even came with a tree that had "thin" and "thick" spacers.
Tamiya also has wheel spacers in 0.5mm thickness (#53646 - Blue) and 1.0mm thickness (#53647 - Red). These aren't in the hex shape, but are a round 12mm washer that fits into the hex of the wheel.
If you don't wanna bother with the washers, you can also get the Tamiya drive hexes in varying widths: 4mm - #53570, 5mm - #53823, 6mm - #53569.
Some other aftermarket companies (i.e., 3Racing, etc.) have the drive hexes in varying widths too.