These
http://tinyurl.com/2jij
fit ZipZaps! The bodies are made of lightweight plastic. I went to a local store that carries all of the Darda stuff and picked one up. Price was $8.00 but they just happened to be having a 20% off everything in the store sale, so I got it for $6.40 plus tax!
Here are a couple pics I took:
http://www.thebrokenjoystick.com/pictures/speedracer
It will definitely fit the ZZ. I'll just have to cut 2 notches. I'll try and finish it up tonight and take more (better) pics.
From what I can tell, most of the other Darda cars should fit too. Some of the cars are very low (like the F1 car) and those won't fit because there is not enough clearance inside. From what I can tell, the Ultra-speed car bodies are made of plastic and are the right size for ZZs. Some of their other cars might work too if they are not die-cast. Here is a listing of all of their cars
http://www.onlytoys.com/darda/cars.html
All of the Ultra-Speed cars are listed there as well.
BTW - you can find some of these on ebay (Speed Racer too) but make sure you are not bidding/buying the die-cast ones!
Brian