can i buy an iphone 5 factory unlocked from usa?

Not yet, it typically takes around one to two months for non-contract locked and unlocked phones to be sold in US.

There's an issue with that, as no one is yet sure whether you'll even be able to ever buy an "unlocked" phone. It seems that the 4G/LTE requirements are hardcoded into each phone, meaning that you can't just switch from one network to another. Don't know if that'll affect 3G or not, but it means that you can't take a 4G phone from the US and run use it on a different network in another country.

Hopefully Apple has an answer to this, as it's a sore point with a lot of people, but so far they've been silent about it.

Update: Apparently the LTE phones come pre-unlocked. Meaning that while you can't change the 4G network, all you need to do to access another 3G network is to switch out the Sim card (which may involve cutting your existing Sim card). That's a fairly delicate process, so you'll want to read up on how to do it, but at least the Verizon iPhone 5 has been repeatedly documented to work on the ATT 3G network.