This is the bird's eye view of how shorting works: When you place an order to sell a stock short, your broker attempts to grab the desired number of shares from any accounts of its other customers and makes them available for you to sell. If no other customers own shares of this stock, then generally you are out of luck (It is more complicated like that in practice, but this is just an overview).
Your odds are better if the particular stock has a large float (i.e. a large number of shares that are actually available for trading) and its short ratio is low (which means relatively few shares are currently being sold short). Also, a large brokerage may be more likely to have access to the shares than a small niche-market broker.
The example you've given, Angie's List (ANGI) is a $600M small-cap with a comparatively low float, and though I haven't been able to glean the short ratio, it appears that a lot of investors are bearish on this stock and probably already had the same idea to short it.
There is really no way to find out if a specific broker has shares in inventory available for shorting, short of (forgive the pun) checking directly with the broker.