It's not really a viable method anymore. Twenty years ago, over the counter sleeping pills could kill you if you took enough. Now they use a different formula. It would take like three full bottles to even get close to a lethal dose.
Now, if you could get a prescription for Lunesta or Ambien, that could work, but it's still not a guarantee, and it'll be hard to find a doctor that will prescribe you more than a month supply at a time.