I was born in Poland and came to Canada as a baby.
So you basically are a Polish citizen (unless your parents were foreigners living in Poland) and you have to contact the Polish embassy and ask them to issue you a passport, you will get It for sure but I'm sure there will be a lot of paper work, so finding the birth certificate will definately not be the most frustrating thing.
but that could give you problem with staying in canada
If I can't find it, what is the process to get a replacement?
Not sure but probably you should contact the local Urząd Stanu Cywilnego in the place you were born and ask them to send you a copy.