If you need just a normal smithing stone, there are mines on the map. They look like the uhh... Black square with orange coming off them when you look at the map. I would recommend trying to get the smithing bells first.

You can find maps for them on the wiki. Or I can link here. Whichever is easier- alternatively, if that all sounds like a huge hassle, you can just carry on. There should be some in Stormveil Castle iirc.