TDA only has 10K shares of VXX to short right now. I used to be able to short a lot more than that, but I guess everyone and their mother is shorting it now. Not sure how you will short this when you decide to short it.
That's whats great about the variety of vix related ETNs / ETFs ... vixy, uvxy tvix whatever, you can usually find something to trade long or short ... vxx is not the only game in town (and not even the best since its an ETN but the most liquid of them, so the best for options).
the one time I really WANTED to short bigtime and failed to get shares was when TVIX went into orbit in mar 2012. I found only fidelity had the shares, but by the time I funded my acct (they had a &*(^(&^@#$ 10 day funding delay) TVIX realigned, but of course that was still at $7 pre 10for1 split so I still could have shorted and killed it!