I am from San Diego California.I have a son who was born in Australia. His mother won't let me see him. I am trying to get some kind of legal custody. I need his birth certificate. How do I go about getting one??

Ok.
1. where do you live now

2. where does the mother and baby live now

The visitation rules of that county will apply, if the mother is in the US, the law of that state will apply.

Also in the US if you don't have the birth certificate you can request a DNA test, to prove you are the father and get a child visitation agreement.

You can contact their embassy in the USA Home - Embassy of Australia

I live in San Diego, California.
The mother and child live in Australia.
Thank you for replying. Any information is appreciated.

From the information I read, you can get one though the Australia embassy.