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.