If you did not have it, normally USCIS will ask for it. Or in cases an interview is scheduled, you must bring a copy of the judgment with you to the schedule.
If you were divorced in the US, you can get the judgment at the court granting your divorce. $8+$1.35 usually the cost.
If you were divorced in China, get the divorce judgment notarized by China Noatary. USCIS accepts any documents from China if it's notarized. Remeber: Chinese Government is the only notary in China.