I would just buy another one. This reminds me that I should probably figure out what to do if the Harmony remote gets lost, and also have one backup Directv remote.
I finally found a missing horseshoe in the yard. It was missing for months. Another time I found all the lamenated cards (to operate electronics) in a box for an air mattress - the box was located in a trunk. This same group (with wild kids) also called right after the greet tour and asked where a remote was. They finally found it in a place where only one of their kids would have put it. Sheesh!
Just keep searching in between every single cushion, corner, etc.