why haven't we found a white hole based off of Newton's third law?
Why would we?
equal and opposite reaction to a black hole
Gravity is an attractive force.
exactly. the singularity of a black hole theoretically should have an opposite, Einstein theorized it in general relativity but we haven't ever found one. maybe they are actually stars 
Well it has nothing to do with Newtons third law (that would just be that when a black hole pulls something, that something pulls back with the same force)
As for finding one, we probably already have and don't know it as different from a black hole. Maybe we are in a white hole?