Charles..
Here's the link where you can purchase one.
https://dtops.cbp.dhs.gov/
It cost $27.50 ( if you go standard delivery, a little more for a faster delivery ) and is good for a year.
When you call ahead to let Customs know that you are coming ( As per the eAPIS process ) mention that you don't have a sticker and they will tell you if you can proceed,
Most of the Customs officers will sell you one if you happen to show up without one.If you have purchased one but didn't get it before your trip, bring the letter ( receipt ) saying that you paid for it and that will be fine.
I just got mine for 2014.
Best of luck
Bruno