From the respective Gacha in the shop and playing the world tiles.
During events there are special deals in the Event shop where you can exchange event currency for random emblems.
And then there is Ocica - which has the chance to drop High Tier (4-5) emblems and tanks. Its a very small chance though.
I think I got one 5* Emblem from Ocica, so far, no 5* tank though.