I am looking at a leather (not sure that it is real leather) case with both belt loops and belt clip. I have used this style of case for my previous phones and been very happy the case did not slip off the belt which happens all too often with just a belt clip. I get them on Ebay but I expect they should be available elsewhere.
I learned the hard way, the HTC One and HTC One M9 are different sizes, so when ordering do not just order a case for the HTC One as I did.
I cannot link, but to see an example of what I want, search on Amazon for
HTC One M9 Holster, CellBee® HTC One M9 Premium PU Leather Pouch Carrying Case with Belt Clip Belt Loops Holster
I saw a similar more pricey version in a mall kiosk. They had one with a bit more room than necessary. When I showed him that, he reached in the lower drawer and came up with one that was a perfect fit. He asked $28.00 for it. :-$ When I hesitated, he dropped the price to $22.00. This was at the Jordan Creek Mall in West Des Moines, IA.
My new concern is there is no room for heat relief which the M9 requires as many apps continue to use the CPU and memory when in sleep mode (display off). I carried it in my shirt pocket and the m9 was getting very warm ... a certain cell phone killer over time.