Where on the Internet can I find the various income levels--NOT the mean or average!--of US families?

Answer 1

The Census Bureau - that's their job.

Here is a sample of one their tables on the subject:
http://pubdb3.census.gov/macro/032007/hhinc/new06_000.htm

Here is their main income data page:
http://www.census.gov/hhes/www/income/income.html

You can also get income data based on geography from them.