https://www.bitfinex.com.

Edit: In case any one needs a site for shorting (Disclaimer I lend out on this site)

People seem to love talking about shorting Bitcoin, and a crash wouldn't surprise me all, but given Bitcoin's meteoric rise recently you'd have to be insane to actually do it.

"Markets can remain irrational a lot longer than you and I can remain solvent." — John Maynard Keynes

The best thing to do with bubbles is stay the hell away. Better to put your money in an actual investment with real returns.

Pure intuition and probably wrong. Better odds than as casino to though. And probably just as accurate as any model.