Call Nike, and Adidas. Let them know you are interested in re-selling their products. They will direct you to their distributors or send you a resell packet and catalog. Sometimes you might need to buy a certain amount of products to start with them.
So to say, you cant just buy 1 pair of shoes from Nike themselves. You might be able to buy 1 pair from the distributor.
You have to call and speak with real people from the company. Be careful with middle men. Too many will leave you without any profit and a recipe for failure.