I was wondering this last winter and found these ...
www.jojomamanbebe.co.uk/detailfash.php?type=FASH&code=A8978&proddesc=Long+Cotton+Non-Slip+Rich+Socks &supercategory=&branch=&wcategory=&catdesc=&treecode=TRE00009
But, Jojo's normal socks actually come up quite long too. BTW I've found their sizing comes up rather large.
Hope that helps.