They are thin and far between owing to the good public transfer up there.
Have a look at these guys:
http://www.portoairporttransfer.com/
Have used their sister outfit here in the Algarve.
You can also try the company mentioned in this article on my blog, Quality Tours:
http://goo.gl/52Jhs
I visited their offices last year while up north and was impressed by what I saw.
However, I have not actually used their services.
Their transfer page is here:
http://goo.gl/5aQ5P
