In case anyone is still looking for snow tire to fit stock 18" wheels, I just noticed Tire Rack has both of these: 225/45VR18 XL and 245/45HR18 RunFlat and are the stock tire size - both are low stock.

I have not seen this size available at all on their web site for quite a while and I'm sure they won't last long.

 

I've got them from last year and they are very good in the snow. Not bad at all on dry pavement. I found them to be nearly as quiet and smooth as the stock Potenzas.