It would depend when and where a person died. With the exception of states in New England, most states did not start keeping death records until the late 19th or early 20th century. Death records are usually kept at the state level, but a few county courts kept death records for limited periods of time prior to states taking over that function. Some death records are on-line (or at least the indexes to the records are on-line), but other for other states you would have to make a request through the state vital records office.
Dear Mr. McDermott,
Thank you for posting your request on History Hub!
We suggest that you contact the vital records office for the state where your relative died to request a search for the death certificate. You may wish to note any name variants or broaden your search for the surrounding years. In addition, the FamilySearch Research wiki for How to Find United States Vital Records may be useful.
We hope this is helpful. Best of luck with your family research!