The best source I know of for statistics like this is the Federal Reserve's most recent "Report on the Economic Well-Being of U.S. Households" [1].
It's a survey of about 10,000 people and, as far as I can tell, is the source for many claims like "40% of American adults cannot meet an unexpected $400 expense", which has been consistently true for several years, although it used to be more like 50% in 2013. Note that “cannot meet” here means anything other than “would pay the expense in full before next month”.
[1] https://www.federalreserve.gov/publications/files/2018-repor...