Where on earth are single-celled organisms generally found?

If you are referring to bacteria, and other possible microorganisms, they are found everywhere whether that be the air surrounding us, to abiotic, non living components of the environment, such as water, soil, etc. Generally though, one celled organisms are found in soil. Substantially, it has been noted that a single hand full of soil, contains more microbes than the people in a given country etc.