The Apple MagSafe connector is proprietary to Apple. This is why you are having difficulty finding any: There are no 3rd party solutions here.
That Apple would not consider this to be a scenario work resolving is to me a bit of a mystery: nearly every Thunderbolt Display customer would require such an adapter for their new macbook. I presume they figure customers would rather buy a new monitor?
2 suggested solutions:
1) Fortunately for you, the Magsafe power cable from the Thunderbolt display is Single Purpose: it only supplies power. Replacing it loses no other function. Therefore, you can simply add a USB-C power adapter (plugged into the wall/mains) and run the cable alongside your Thunderbolt cable from the monitor. Simply hideaway the Magsafe cable.
Yes this might be a bit more unsightly than your current two cables, but should be simple to accomplish, and can be as easily well hidden as your current setup.
2) I have found great success in using a Thunderbolt (now UBS-C) docking port, hidden away under my desk. My monitor, external hard drive, network, etc all connect to the dock, and I have a single Thunderbolt (USB-C) cable from the Dock. I have a small USB hub attached behind my monitor to replace the convenient USB ports I would get from an Apple Display.