I am looking for median income statistics per county for the time period from 1930 to 1990. I have found these median income statistics from 1960 to 1990 at the US census bureau. However, I seem to be unable to find any earlier calculations at the county level. Do they exist? If so, where?
I suggest you search IPUMS, which has variables going back to 1850: https://usa.ipums.org/usa/index.shtml
I quickly searched and found that at least, personal income has been evaluated going back to 1940: https://usa.ipums.org/usa-action/variables/group?id=income
But a more detailed search might be even more fruitful.