I am trying to fill my 'Dex and I need the Popplio evolution branch but I can't find Popplio. (Thanks for the help)
Popplio and its evolutions cannot be found unless you have it as your starter, or if you trade with someone. Simple as that.
You can only get Popplio as a starter, from trading with a friend, or wonder trading.
People usually try to IV train their starters so you might find some on GtS from people who have too many of a starter who is willing to trade theirs for something else. Also on wonder trade it is pretty common
Starters are only a one-time opportunity. The only way to get multiple starters is if you trade, or if you breed your starter with a ditto if it is a male, or with another compatible egg group Pokemon if it is a female.