Margin trading is risky in a regulated market and doubly so in the cryptocurrency marketplace. I would advise against it, but as another sibling comment mentions, Poloniex would be your best bet.
Be careful though, as Bitfinex, another popular margin trading website, was hacked and lost $60 mm a few months ago. Polo has also been hacked before, although they incurred a much smaller loss due to their hack.
When you margin trade through your brokerage, you don't have to worry about the firm shutting down and/or the US government not repaying your money. With these websites, there's a very real chance it'll disappear overnight.
