<a href="https:&#x2F;&#x2F;www.bitfinex.com&#x2F;" rel="nofollow">https:&#x2F;&#x2F;www.bitfinex.com&#x2F;</a><p>Large, relatively trustworthy exchange. Costs roughly 3.3% APR to borrow $1m worth of BTC for shorting right now: <a href="https:&#x2F;&#x2F;www.bitfinex.com&#x2F;credit&#x2F;index&#x2F;btc" rel="nofollow">https:&#x2F;&#x2F;www.bitfinex.com&#x2F;credit&#x2F;index&#x2F;btc</a><p>Edit: corrected the interest rate