you can contact sprint directly, yes. it takes about 6 weeks to get setup and in their system before you can start ordering phones. they will provide you with all P.O.P. displays, dummy phones, brochures and tons of extra stuff to setup your store.
with dealing with sprint directly you have to maintain a quota or they will drop you. you can use a master agent and still sell sprint though. you don't have to worry about quotas or anything. If your looking to setup a store selling sprint, their are a few different options of doing it. if your looking to do a mall kiosk/cart figure investing about 25-30k. if your looking to do a standalone figure more into 55k