Find an appropriate lens cover on parts4repair.com, amazon, etrade supply, or other online sources. Reading various blogs there are a few that people seem to be using. Below are the common lens covers that are being used:

#1 Option: HTC One Lens Assembly

#2 Option: HTC One X Camera Lens

#3 Option: HTC Evo 4G Camera Lens

This guide outlines using all three and weighs the pros and cons of each

At the time of writing this the original HTC One Lens assembly was hard to find, but it's the best option for lens replacement. So if this is still the case then the HTC One X lens is the second best option and the Evo 4G is the third best option.

On Amazon, eBay, eTrade supply the HTC One Rear Housing is being sold and contained on this is the lens assembly, this can be purchased and the lens assembly removed from this.

The downside of options #2 and #3 is that the lens protrudes from the back of the phone and may lead to scuffing of the lens. If a protective case is on the phone then scuffing of the lens is not a concern and any of the options will work just fine