If you would like to withdraw your support of the petition that was previously filed, you may send a letter to USCIS stating that you no longer wish to sponsor your ex-husband. You can include a copy of the Divorce Decree. After that, you no longer need to worry about any of it, as you a no longer responsible for the sponsorship.
Write to both the USCIS office in Chicago (National Benefits Center), and the Houston District Office. But you do need your I-130 receipt notice, otherwise the USCIS will not be able to match up your case with the file. So ask your lawyer for a copy of your petition. Since it is your petition, you have a right to that petition. So write a letter to your lawyer asking for the file, and you should be able to get it.
The attorney has taken the reasonable position that he represents both you and your now ex-husband. This means that he cannot withdraw the petition, because he will hurt his other client, your husband. He also believes that you should have gotten the receipts from the U.S.C.I.S., as well. Your attorney's position may be that if gives you the I-130 receipts that you should already have, knowing that you will withdraw the petition, then the attorney may be accused of prejudicing your ex-husband.
The red, white and black I-797 receipt forms contain the file number for the case. Nevertheless, the I-130 petition is technically your petition, so you should get copies in the mail and from the attorney. If you have trouble, consider scheduling an InfoPass Appointment or going directly to the Information office of your local USCIS District Office to secure a copy of the receipt number for the I-130 petition that you filed on your ex-husband's behalf. The Information Officer should be able to tell you how to withdraw the petition, as well.
You should send a certified copy of the divorce order by certified mail to the attorney as a precaution. Also, confirm your conversation and let the attorney know that he should take no further action on the petition. If you desire, you may also tell the attorney not to represent your ex-husband, as well. The attorney should have already withdrawn from the case in this sort of situation.
Be careful. Until you are certain that you have successfully withdrawn your petition, you may still be responsible for providing your ex-spouse at 125% of the federal poverty guidelines and/or be responsible yourself for filing a change of address (as an I-864 affiant) within 30 days of any move. Check with an experienced immigration attorney to be sure.