Rarely from Gems gacha, you get Eidolons or Weapons with +1 on them. Whatever you feed that one on, will gain that +1. And when you give a +1 one to a +1 one, you'll get a +2 one.

You can also give weapons with + to Kamihime, but you shouldn't. It's better to have them in weapons and give the bonus +3 Atk +1 HP to all five of your team rather than just one Kamihime...

More commonly the +1 things can be found from Raid gacha (there will be a raid in a couple of weeks) and especially Premium gacha has loads of them.

Oh, and feeding a +2 one will transfer the +2, if you so want. You can only lose the + stuff if you use that item as Limit Break material.