After you and your fiancé marry you can complete a stepparent adoption. You must try to get consent from the kids' biological father. Because you do not know where exactly to find him, you can file a Petition to Terminate Parental Rights with proof to the court that you attempted to locate their birth father to notify him of the adoption to no avail. If the court is satisfied that you made your best efforts to notify him and he did not respond, his rights can be terminated and you can move forward with completing the stepparent adoption.
Common ways to prove to the court you attempted to notify the birth parent are keeping track of your records of completing some (or all) of the following:
1. Sending a letter via certified mail with return receipt requested to birth father's last known address notifying him of the adoption;
2. Calling friends/family of birth father and asking if they know of whereabouts;
3. Review of phone directory in the city you believe he could be living;
4. Internet search for any contact information;
5. Contacting the Department of Child Support Services;
6. Checking voter registration records;
7. Contacting the County Recorder's Office.
I do suggest you contact a local family law attorney to assist you with preparing the Petition to Terminate Parental Rights. It is not a court prepared form, but rather a free-form drafted pleading. You will also need to attached as exhibits the evidence that you made your best efforts to contact and provide notice to birth father.
I hope this helps!