How would I go about finding my 2 half sisters I never met?

You've probably already done some looking around on the internet, and if that has not provided you the information you need, you'll most likely have to escalate matters and contact a private investigator. Following a trail like the one you describe is time consuming and difficult, so you will need to be prepared to be patient with the process and invest a lot of money in it. Only you can decide how important this search is to you, and if it truly matters, the cost won't bother you. A reputable investigator can tell you how things will most likely turn out, and can give you a rough idea of what the cost will be.