If you want a UK car then you'll struggle to get a yellow as these were GT4s only. There were a few changed hands for silly low money so you never know....
Azure should be easier to find but they are thinner on the ground as is black. Should be plenty of GM cars.
Ebay, PH, AT and here. Gumtree I guess too. At least if we know what you're looking for someone here might give you a shout if they see something.