Anyone willing to rent a car on STX? Please help!

I am a 26 year old single mom to my 2 year old son and I am looking to rent a car for up to 3 months. I was hoping I could avoid going to a rental car place by possibly renting a car from someone who is off island or just has an extra car they aren't using and could use some extra cash. If not, does anyone know of an inexpensive place that rents cars for that long? Any information is greatly appreciated!! Thank you !!!