I've got one from very early on in the game and I've seen it 3 times so I don't think it's rare but it's semi-rare.
BP Falken RX7 is rarer I think as I've never seen it in the UCD but I won it in the 1000 Ticket.
I'm quite sure you win one of these...if not, I dunno where mine came from xD and it's not that rare, seen a few. It's the only car I ever use for the drift trials, nothing beats it with my setup...