Names like Nike are distributed through tightly controlled authorised dealer networks, not through general wholesale where any trader can easily access the stock.
You can find these types of shoe loads from certain liquidation (surplus) suppliers, however they will usually be older stock, so last season or beyond. Having said that, they can still represent seriously good value at the price these loads can sell for.
A good place to start looking would be www.viatrading.com, they do offer good value shoe loads at times.