As I said above, the gear that can use them is all crafted gear that gets an Exceptional/Critical result. It looks just like the standard gear but has a single augment slot...
That said, I've also heard there might be a display bug in the GTN that makes that augment slot not show? Or was it random second stats don't show? So perhaps they are up for sale and just never show their slot until you buy them.
I get display bugs for crafted gear second stats, like power, defence and so on. They don't show up in comparison. Also, the comparison of the other stats is buggy, sometimes shows as a loss in stats, even though it's identical, or whatever.
Anyway, I found this thread because I crafted a piece of exceptional gear, and wanted to know where to find augments. Now I know. Thanks all.