Hi Mj
12mm hex is the standard size for 1/10th scale RC cars. Tyres however do vary. I have done a quick google search for uk sellers only and have found a couple of shops that carry spares.
One such example is here:
Hobbystores Aircraft
(If the page defaults to the Talon Model as it does sometimes, the tyres are still in that list, your car is listed down the left side of the page and all the spares are there)
The tyres are listed as Tyre Blocks for some reason but they are the ones you need....Probably a good site to bookmark for future reference.
Regards....Mark