There are griffon vendors in various locations on Test right now -- I found one right at the newbie spawn-point in Rindol Field, and was finally able to see what I'd missed in beta.
One word: OMG!
Crashes and other dangers notwithstanding, that was an absolutely awesome experience.
Note that these griffons are rentals, judging from the tooltip text, sort of like the horses crafters can rent, so I assume they'll disappear from inventory after a while. Not sure if one can get another after that.