When I got my M7 last summer, these were $1.99 each on Amazon. I got like 10 of them, figuring if I didn't like the color of the one I wanted originally (blue) I could easily switch for something else. Turns out I hated the blue and really liked the black. http://www.amazon.com/SUPCASE-Premium-Smartphone-Protector-Included/dp/B00DPIL8ZU/ref=sr_1_7?ie=UTF8&qid=1397999802&sr=8-7&keywords=m7+tpu+case

My wife bought me one on Ebay that I use now, just because it's cool. It's a Storm Trooper with a mustache. I've also seen some really cool wooden, engraved ones ( http://toastmade.com/HTC_One.html ). The phone doesn't need a case at all, but I like the design.