Combined Lego Super Heroes (Need Advice)!

Coming to this looking for some help! My 3-year old got a couple small Lego sets for Christmas & loves them - which makes me happy. He is also obsessed with super heroes so I wanted to buy him some more. Question: Is there an easy way to buy a set/group of JUST Lego super heroes? They don’t need to have any crazy accessories or be part of a broader build set - I’m just looking for a way to buy a dozen to 20 of them. I’ve seen sites like Walmart sell sets that look exactly like Lego but they...