Unfortunately the only way to see Popplio if you choose Litten is through trade. What I recommend is that you breed your Litten/Torracat/Incineroar with another Pokemon (Ditto if it's male.) and get a Litten egg. Once you hatch it put it up on the GTS asking for a Popplio and you have a good chance someone will trade for it. Other than that you can just, like you said, hope you get one off of wonder trade.