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.