Illumination Supply carries the KeepPower 1x 18650 case.
item on Illumination Supply’s site
item on KeepPower’s site
I purchased two and have been very happy with them. They have not yet seen hard use (backpack or pocket), but I have plonked them around on my tables and desks. I am pleased with the clasp, it has a very positive action when it clips or unclips. The feel of the plastic is good, etc. They easily accommodate my longest cells (NCR18650B with a cheap protection circuit). Fitting long cells and having a good clasp are really what seals the deal here. I have a lot of 2x18650 cases from varied sources (DX, FT, MTN Electronics, others) and none of those has a clasp that feels anywhere near as secure as this.
On the FT interesting items thread Chloe recently brought up these silicone carriers, you may want to look at them as well depending on your application.