Or more generally, go to the "Diecast & Vehicles" section of ebay (under Toys & Games") and search for the vehicle type you want. Then use the "scale" tick box on the left to narrow down to a size you want (it is always worth clicking to get the dialog box showing all available scales to see what your options are). Some sellers don't bother listing a scale or size, annoyingly enough.
Even then, many manufacturers and sellers get the scale wrong, so ask for a measurement of the model's (not the box's) length or width, if it isn't given. Then get the length/width of the real thing from Wikipedia and check if the scale really is right for you.