Is there anyway to get the robes and hood and what-not from the Greybeards? I'd imainge they would have a few spare lying around, they cant wear the exact same ones forever can they? Unless they have a shout that washes themselves.

You could always try killing them, don't know how that would work out

Got a feeling they might be unkillable since theyr'e essential :[

You can't. They aren't even marked playable in the PC version.

Would be surprised if console wasn't the only way, reasonably sure that all Greybeards are essential.

Just check Skyrim Wiki, and yeah, all Greybeards are essential. You could try getting Pickpocket to 100 and stealing equipped items, but I'm not sure that they'll be available in the inventory - like the standard armor of companions.