Probably the warmest socks you'll find are Steve Ashton's Heritage Hose at Freedom Kilts. Not cheap but worth the price.

The Scottish Tartans Museum has several styles and prices to choose from.

Hamilton Dry goods has wool hose very inexpensive and very "plain".

And if you are up fro a little drive over to Rocky at USA kilts in Pheonixville, PA, they can take care of you also.