I looked online on different websites for a gasket that I need. In taking off the oil filter adapter on my 2003 Ford Mustang 3.8L V6, I found that the gasket that goes between the adapter and the engine is in need of replacement. Seeing as I could not find tbe gasket online, I was wondering if I could use RTV instead. Probably a terrible idea, but I was just wondering if it was doable.
According to This ford specific parts website you are looking for part number 6C639 (or possibly 6600 if I am misunderstanding your description of the part) Note the square and circle in the top left corner used to denote which gasket is which. Don't let those prices scare you, searching for the same part number on Amazon finds it for 10 USD.
What I'd suggest instead is: if it's a paper gasket, get a sheet of gasket paper (available at most auto parts stores) and very carefully trace the old one onto the paper (be sure to use the finest-tipped implement you can to avoid making the new gasket slightly wider), cut with a razor. If it's a rubber o-ring, they are available in various sizes online as well as in stores. Measure the inner and outer diameter and search using those dimensions. If it's an awkwardly shaped rubber gasket... God help you.
Short answer - no.
Mostly RTV is just not meant for that kind of pressure. In a pinch, sure - short-term, but not as a fix.
As Solar Mike said, you can get it at the Ford dealer. This same basic engine was used in a lot of different vehicles over many, many years. The engine model was only replaced in the Ford lineup in the past few years.
You don't want to use RTV, it will never make a good seal, and if you did happen to get it to seal, it could fail at any time, leaving the life blood of the engine out on the pavement. You may not know it until engine damage occurs. Just replace the gasket and call it a day.