Maybe you still are waiting for a invitation code from other Oneplus one phone owner, because Oneplus official still only release about 2000 invitation codes so far. But if you want to pay about more $100, you also may buy it now from unofficial online shop. Oppomart is one of them, and a free official cover with each Oneplus one phone recently.
Oneplus launched 2 official protective case on their Chinese official shop now: a leather case and a skin cover. The skin cover is made of flexible PC plastic, it has 5 color option: white, yellow, orange, green and clear. There is Oneplus logo on the back.
There is plenty of unofficial protective case and cover on oppomart, if you want more, just visit here: Oneplus one case [source]