Hi Tunmore and welcome to The Registry. Hopefully you will find it a useful resource.
You are correct in thinking the areas are named the A and C pillars. Unfortunately these are a common rust spot on cars that reside in wet climates.
I would recommend trying to source some from a warmer climate, but it is worth considering that most cars are parted out as they are not financially viable for restoring, which usually means they are rusty and will probably suffer from a similar issue. That said, a majority of members are from the US and warmer latitudes so you could be lucky.
Why not place an advert in our classifieds section and see what happens. You could also try our mailing list as there are some members that prefer the list to communicate.
If you do not have any luck, any good metal finisher should be able to fabricate a repair. From memory the front A pillars are made from three pressings welded together. Somewhere on the site there are some photos of a cross section of this area, it may help you. I will see if I can dig it out.
HTH
Mick.
PS don't forget to post some pictures, and if you're feeling inspired you could also keep a blog of your progress