usually better lines ask for a press kit of your store...w/information like the size, the 'look' of your store, other lines you carry, volume of sales, as well as whether they have their own store or other stores near your vicinity selling their products.
it's a very difficult task to get really good stuff off the bat. you would most likely to focus on the smaller names that is about to hit it big. they are more willing to work with. also most places will be more willing to let a new vendor carry their accessories and shoes line but are more difficult to get the clothing.
best way i think is to start with whatever line you can get and go to the shows and get to know the sales staff esp since most of them move around the different company so if they love you, they will give you the products.
therefore, it may be easier to buy a store already up and running or work for a boutique for a bit until you get to meet the sales rep and such. i wouldnt suggest opening up a store from scratch when you dont know anyone in the industry (but that's my two cents)
good luck!!