You can craft many good sets of your own, see this site for an overview:

elderscrollsonline.wiki.fextralife.com/Sets

For the crafted sets, you need to know a certain number of properties. If, e.g., you want to craft medium boots of the Hunding's Rage set, you need to know six properties on medium boots. It doesn't matter which properties.

Other sets can be obtained as drops in PvE (open world as well as dungeons of all kind), via the chests you receive keys for by completing the undaunted pledges, or in containers that you can buy for AP in Cyrodiil.

As a good starting point, I suggest self crafted armor and weapon sets since the other ones can be hard to get your hands on.