A. If you’ve done all your searching online thus far, don’t worry about being stuck: You still have plenty more avenues to explore.
If your great-grandmother died after 1936 and had a Social Security number, she should appear in the Social Security Death Index, or SSDI (search multiple versions of this database simultaneously from Steve Morse’s One-Step site). The SSDI lists the deceased’s last residence, where you can check to see if she died or was buried.
Try to request a death certificate from the vital-records office of the state where you think she died. Every US state was issuing vital records by the 1920s, so you wouldn’t need to know the specific town or county to get the record. See the National Center for Health Statistics’ Where to Write for Vital Records Web site to learn the address, fees and ordering information for each US state.
Check Great-grandma’s hometown newspapers for obituaries and death notices in the time frame you believe she died. You can identify newspapers published during that time, and which institutions have them on microfilm, at Chronicling America.
Research the husband. You know from your census research he died between 1920 and 1930, and I’m guessing you also know where based on where they lived. Use this information to try to get his death certificate. Check newspapers for his obituaries, too. By identifying the husband’s burial location, you can find out if husband and wife are indeed buried together.