You first posted this under Ohio. Now in California. State laws make a difference. In Ohio an attorney has the authority to issue a subpoena.
If you do not have an attorney, the clerk of court will have to issue the subpoena as a lay person may not issue one. Most clerks will require that the judge be involved, so simply seek leave of court in your pending case to proceed.
If your ex's attorney was supposed to do this, then they can be held for contempt. That is where you can start. YOu can also file a motion to have you ordered to get the QDRO done. A subpoena isn't what you need to do. Seek legal assistance on this.