Interactive Brokers provides data showing borrow availability and the rate you'll pay to borrow the stock. Using this data however is a bit clunky. Fortunately IBorrow has come along allowing one to quickly pull up and visualize the data.
For those looking at option strategies, which broad based etf's to short, or other strategies, knowing the quantity and rate of shares available can be important. I suggest you give it a try.
ht: Glenn Chan