amazon S&S is the cheapest way to go in general.
From time to time there are specials at office depot or kohls.
For example, a few weeks ago, OD had 100% back in worklife rewards on K-cups. I bough about 12 boxes for 9.99 each for a total of $120. In a few months, once the quarter is over and they pay out, ill get an OD gift card for $120 making my out of pocket the taxes only. Ill use the GC on things i need, or on FAR items and "cash them out". Its a bit of a process, but free is free...