documentary film project and would like to know if anyone could
suggest a source to find out if my particular idea has been filmed
You might want to look under the TOOLS section of Transom.org. Transom.org is an experiment in channeling new work and voices to public radio through the Internet. The TOOLS section includes info and guides covering the technical aspects of recording/ interviewing. There is a "Mini-Disc Guide" guide as well as "Remote Recording Survival Guide."