Can I buy an iPhone 6 at the Apple Store with iOS 8.1 installed for jailbreak purposes?

Im going to USA i want to know, to buy the corresct one, and if have anything in the box that can indicate me...

Depends on the inventory turn-over at the store you visit... chances are, their inventory of devices with 8.1 on them has been sold through...

I"m not sure there is any way to know the exact version of iOS installed on a boxed iPhone 6. Depending on when the phone was manufactured, that version could change.