If you can't provide enough proof materials, you can try to seek help from the local public security bureau, police station or community neighborhood Committee. They may provide relevant certificates or assist you in handling the death certificate.
In addition, if grandparents are buried in a hospital or cemetery, they can consult relevant certification materials from the hospital or cemetery. If grandparents are buried at home, you can consult the local village Committee or religious groups and other departments for relevant supporting materials.