Where can I find a 12in (L) x 12in (W) x 16in (H) corrugated box not sold in a bundle?

Try the UPS store. They have boxes in different sizes that I believe they sell individually. Also try storage places or moving places.

Check out your local rental moving-truck places like U-Haul, Budget, etc. They usually sell boxes by the size and individually.

You can also check out your local phone book for moving suppliers or box suppliers (or google those words along with your zipcode or city).

Or learn how to make your own box from corrugated cardboard, or to cut down a corrugated box to a smaller size so you can have any size you want. You'll want a matte/utility knife probably, strong tape, and perhaps a dull dinner knife or "bone folder" to make creases before folding.

Container Store