There are always eBay auctions, but recently one trio of cars was selling for $65! Before you spend the big bucks, the diecast toys should become more plentiful in the stores over the next few months and will distributed at least through the holiday season.
So if you can't find a Sarge or Lizzy locally, be patient and wait a month or two. According to a Mattel spokesman, the cars should be showing up locally.
The different CARS can be a bit confusing, so to help figure out what's available to complete a set, take a look at the Mattel Character CARS assortment.
Several of the cars are not pictured: Leakless, Nitroade and Dinoco McQueen.
Also read about CARS Movie Moments Assortment.