What can one do, if one's husband refuses to go into mediation, or
refuses to get a post nuptial agreement written, prior to Legal
Separation?  Also, I live in a suburb of Los Angeles, in Agoura Hills,
Westlake or Thousand Oaks; where can a couple get mediation and/or
pos-nuptial agreement without first signing up for Legal Separation? 
And hypothetically - if one does sign for legal separation, can they
change their mind during possible mediation and/or post-nuptial
agreement?
For example:  What attorney(name, address and phone?) would do this?

You may want to simply get a free referral from a family law attorney.
 You can check with your local bar assoication for this:

http://www.lacba.org/

Good luck