Anyone with Armorcrafting can make you a decent shield. Your best bet is to find someone with Armorcrafting and get them to make you one. It shouldn't be very expensive at all (I'd almost expect people to give you one for free, but ye never know).
There is a shield vendor in Freemarch, according to the database. However, this would require going into enemy territory to find a vendor which may or may not be accessible by Guardians. (DB says its Neutral, but ye never know...)
Edited, Dec 26th 2011 12:19pm by emallson