what do you mean "where" are? if u don't see them posted by some "popular" guys, it does not mean that they don't exist and are not viable.
what does mag mean in a build for a tank? to tank with your magicka? every serious tank uses ice staff yes, but also shield. You block with both weapons, depending on your currently active bar.
If you mean a build that specifically uses frost staff (probably as only tanking weapon) and only uses magicka for everything, why wouldn't that be an option? you can make a build yourself, it's not like there's any rules as long as you do your job. What would you need stamina for, anyway?

the mythic chest is overkill in pve, you regain magicka from heavy attacks, which is beneficial also because now it gives you a better absorb shield. And you don't spam abilities like dps do, to need a constant flow of resources.

sets are not really important, it is more a matter of what you want to use. If you want to min-max then it depends on what your group needs and what the other tank or healers are wearing. For example you can wear the mythic chest, yolna backbar, alkosh front bar, and a monster set, and your last piece will be a Trainee.
As you describe the chest, you intend to be perma blocking with a frost staff and regain mag with the mythic chest, but as a tank built you will need that much magicka to block only for large packs of ads which a full magicka pool at the start of the fight should be enough.

https://xynodegaming.com/natures-bounty