In order to subpoena an individual or send out formal discovery requests, which is what interrogatories are, you need to be in a formal litigation situation.  It is unclear to me what your current situation is.  If you are currently in formal litigation, meaning a case has been filed with the appropriate court, you can solicit testimony or informatiaon this way.  A subpoena is the tool used to request that a witness appear at trial and offer testimony.  Interrogatories are written questions sent to parties or witnesses to a matter.  Generally, interrogatories are sent to parties to a lawsuit and not witnesses.  You have to keep in mind that when you subpoena his girlfriend or family, they are likely supportive of your ex and the testimony you solicit from them may not be favorable to you.  Unfortunately, in a divorce situation, ex spouses make derrogatory statements about each other all the time.  This comes with the territory of divorce or a relationship ending poorly.  Often times there is not much that can be done and people generally accept that ex spouses are going to say bad things about each other.  If the comments your ex is making about you is not effecting you in a financial way or having an impact on your custody or support of your children, tring to pursue some sort of action against him for talking bad about you is likely a waste of your time and money.  If he is truly in contemp of a court order, notify the court which issued the order and have them resolve the matter.  It is in your best interest to seek a consultation with a family law attorney in your area.  He or she can hear all of the facts of your situaiton and help you make a more informed decision with respect to your legal options and what avenues to pursue.