I'm inclined to say Peterson themselves will do stem replacements of their own pipes, in fact I also want to say they'll do it in the US as well. Don't quote me on that just yet, I'll do some googling in a short bit and post in here again.
Edit: Here ya go, straight from the Peterson website itself. http://www.peterson.ie/pipes/faqs.html#replacementmouthpiece
I'm also sure LL or any other skilled pipe repairmen you can find could do it as well. But as far as I know the wait-list for repairs is super long, somewhere in the neighborhood of 12 weeks last time I saw a post.