Usual thing, 1/43, 1/50 etc all work, just don't mix models re: scales, ie if you have a 1/43 hummer then don't get a 1/50 hummer, maisto do nice sets of 1/43 private cars that appear here in Lidl for £3.99 for a box of 4 or 5, good value.
As do the supermarkets, though quality can be an issue. 99% of my cars are 1/43, with the odd 1/50 truck an ambulance and hummers from Fenris Games which are 1/50 -1/55 I think, (Ian help me out here!

). Also hotwheels bikes, choppers etc scale up well for 28mm gaming. Don't forget as well that as Zizi says, 1/43 scale really means nothing in the cheap diecast car world, there are still pretty hefty levels of artistic licence when it comes to sizing, best thing to do: go with what works for your table:


Cheers
The Commander