as has been said charity shops or car boot sales BUT get your kids to buy the cars ....otherwise the seller will think you are a collector and will try to charge you a fortune....
Heres how it goes -
Dad: "Son werent you looking for some more cars...."
Son::Yes Dad, can I have my pocket money now?
Dad: Here we are son £2 - wonder what we'll find in this box...if you are good the kind man might let you take a couple extra
Dad & Son rummage...
Secure six good cars - right scale -
Dad: see how much he wants from them son?
etc etc