I am looking to open a small magazine shop in New Orleans and had some questions: 
How do I find my merchandise? Is there a way to compare these
agencies/vendors? Are there specific publications dedicated to this
type of business? Is there area-specific market research available to
me? Thanks!

The only "magazine shops" which I've seen that appear to be making
money are pretty large and carry a lot of rare publications, mostly
small-circulation exotic comics and lots of them. Otherwise you are
competing with every supermarket and 7-11 as well as publisher's
clearing house.

Just a thought. We sell our local magazine on Amazon.com, by
subscription, and through local stores of all kinds - the deal is that
the person who gets the store  to carry it gets 1/3, the store gets
1/3, and the publisher gets 1/3 and we pick up unsold copies when we
deliver the next issue - no cost to the store to carry the
publication.

You should read archived copies of Circulation Management at
circman.com to see the publisher's side.

Hope that helps.