You're in luck, as a load of new SCX Nascars were recently dumped onto the market at low prices and there are still a lot left for sale. Sadly for me, I bought most of mine at full price before the mass sale.
has all those shown below at £15.99 each. (available on their special offers page)
Pendle Slot Racing
still has a few at £10 each.
Time Tunnel Models
also have some at £11.99 each
There are more liveries available at full price, but if you want those cheap, you'll need to use eBay and be patient. eBay is probably your best bet for Indy cars.
also has two Scalextric Nascars for £23.49 each.