I would like to find my biological father, but do not know his last name. I know where he worked, when my mother got pregnant with me. Any advice? Other than hiring an investigator?

First thing you need to find out is his last name. Maybe you could find some of his co-workers and ask them for his last name. Then it would be a lot easier to find him. Once you have his name you can go to:
WAATP
And find more informations about him there. But for the beginning, focus yourself on finding his last name...

Not a bad link,

But yes without a last name, there really is no chance. When you ask your mom, what does she way, surely she knows his name