Now why would anyone want accurate frequency 50/60Hz mains power? Really the only application is mains powered synchronous clocks. The old sort many of us used to have in our kitchens and elsewhere around the house. These days superseded by crystal-based oscillator versions driven by a 1.5V AA cell. Generally they do an 'OK' sort of job, but it's annoying to have to adjust them every so often as they drift.
In most of Europe, the US and many other countries the mains frequency is held to very tight tolerances long-term. Where this isn't the case and you want to run an old synchronous clock there isn't much you can do except adjust the clock as often as needed. Many rural areas here and on some of the islands the power is derived from local diesel generators which aren't time-locked. A clock could easily gain or lose 30 minutes a day! It only requires a 2% error in frequency to get that result! i.e 51Hz or 49Hz for a nominal 50Hz supply.
This also will apply to more modern electronic clocks that derive their timebase from the mains frequency (e.g. bedside clock radios/alarms). Though admittedly they are few these days.
One source is a Temperature Compensated Crystal Oscillator. Not expensive and the few 32.768kHz versions I have tried are pretty good, say within around five seconds a month.
Of course the ultimate is your own Cesium or some form of Atomic clock, no, I can't afford that either. In the context of this GPS (or more correctly, GNSS) is the answer - unless you're in extreme polar regions. Though that is being/has been addressed.
This all started with this item on Rod Elliot's web-site...
https://sound-au.com/
The site is 99% about audio and more than a few other things, but Rod also has an interest in preserving old clocks. It's worth a look around, I've no connection with the site - yet, maybe in the future though, via contributed items.
The item that created an interest was this...
https://sound-au.com/clocks/freq-changer.html
This device changes your (accurate) 50Hz to 60Hz, or vice-versa...but what if your 50/60Hz isn't accurate as I mentioned above?
Now, I have no idea how accurate time-wise my local mains is here, so I intend to measure it first to find out.
Updates as and when...
Steve A.