As always, Spigen are producing quality cases for all the major smartphones. I always like Spigen cases, I have used quite a few of them and in most cases I was very happy with what I got.
This Neo Hybrid Series Case for HTC One M9 is just stunning. It is slim and light but still provides and excellent protection for this device.
Anther great option that you might find very interesting if you are in a quest for a thin case is this Ultra Slim Fit SoftGel Flexible TPU Case for HTC One M9. I think that this case is just perfect. It is not expensive at all and it will definitely perform an awesome job in protecting your device.
These two cases and many other different styles of protective accessories for the M9 can be found on many retail stores like Amazon, ebay and others. The most important thing to remember is that finding the best case doesn't necessary mean finding the most expansive one. It even doesn't have to mean that you need to find the most protective case of them all.
The secret of finding the most amazing case is just getting the right one for you. For your personal style and your own needs.