Do self replicating, independent RNA strands occur in nature? They are often used in laboratory experiments but I have not heard of any wild populations. I mean independent of other organisms and able to reproduce on their own in the environment. I.e., not just viruses or transcription RNA.
They probably existed in the beginning of life (on Earth or somewhere else). But any free RNA now would be almost immediately devoured by bacteria or other organisms.
Bacteria are ubiquitous. That's one of the first things you learn in a microbiology course. We do this experiment a few times: open up a Petri dish far from a Bunsen burner, and in a few days you can see how many bacteria and/or fungus colonies have grown in the Agar. Any good microbiology book will show you this. I have none right now with me, but have just found a few links by searching for "bacteria are ubiquitous" on Google: Understanding Bacteria, The Purple Phototrophic Bacteria, Bacteria in Agrobiology: Plant Probiotics and Public Health and Infectious Diseases.
As for the science studying the origin of life, I recommend The Fifth Miracle: The Search for the Origin and Meaning of Life, by Paul Davies.
Kind of; they're called viroids. If you mean RNA that also catalyzes it's own replication independent of environment then I'm not aware of any that exist in nature But one could make the argument that all 'life' is simply self-replicating ribosomal RNA which exploits proteins to do other functions and DNA to store it's information more stably.
It's a bit arbitrary to exclude other cells as an environment though.