What city in the U.S or outside of it is self-sufficient enough to successfully be a city state? An example might be D.C but is hardly

New York City encompasses 5 counties and has a budget larger than any US political entity other than the federal government and the states of California and New York. It was proposed c. 1970 that it become the 51st state, though that never came to fruition.

We could here in Toledo. We have a lot of industry, two oil refineries, an electric plant, scores of produce and animal farms and two major Interstate Highways.

I was going to suggest NYC or LA. 
Maybe Tokyo or Beijing. And what about Singapore?

The Vatican doesn't count because, although it is already a city state, it remains so only by the wealth and power of the Catholic church.

No city is self-sufficient. Every one trades with other entities. Neither has any city in history ever been self-sufficient.