Where can I get guardian shields?

I was watching videos for tips to defeating Guardians in BotW and I found this video of guardian being defeated purely by reflecting damage back at itself. I know that you can do this with a well timed parry with any shield, but with the Guardian Shield it looks like you don't have to parry at all.

The only problem is, where can I get a Guardian Shield? I got one from a minor test of strength in a shrine, but it broke. Where can I get them reliably?

Find me a Guardian Shield

According to this wiki, Guardian shields can be reliably found at both the Soh Kofi Shrine and the Pumaag Nitae Shrine after completing "A Minor Test of Strength".

Alternatively, this wiki lists the locations where Guardian Shields more generally can be found.

Guardian Shield: West Necluda, Lanayru Great Spring

Guardian Shield +: East Necluda, Faron Grasslands

Guardian Shield ++: Gerudo Highlands, Akkala Highlands

In all of these areas, the Guardian Shield can be obtained by killing Guardian Scouts.

As I mentioned in this answer to another Breath of the Wild question, Guardian Scouts will respawn after a Blood Moon event.

So strategically, I would visit the areas listed above, and make a point of targeting the Guardian Scouts, as it are these scouts which will drop the Guardian shields you are coveting. You can read about the strategy for efficiently defeating Guardian Scouts here. These enemies (both inside and outside of shrines) will respawn after a Blood Moon.

The regular Guardian shield (which is what is pictured in your screenshot) does not automatically reflect most Guardian beams. Only the Guardian Shield++, which is only obtainable from Major Tests of Strength, can reflect Guardian scout beams - that is, only the lasers from the Guardian enemies in shrines. Lasers from Decayed, Stalker, Flying, and Sentry Guardians will still only be deflected sideways, and your shield will break if the durability is low enough. Source: Zelda Wikia, GameFAQs

If you want a shield that auto-reflects even the stronger lasers, you'll need to get an Ancient Shield from the Akkala research facility. It's quite expensive though, costing 1 giant ancient core plus other resources, and it can only auto-reflect 5 lasers before breaking on the sixth (Source: GameFAQs). My recommendation is to learn the timing to parry the laser beams, as that will save a lot of shield durability and won't waste resources.