You can also see if there are any pregnancy resource centers near you like birthright. They usually have a lot of donated formula and diapers and stuff. Talk to pediatrician about making the transition easier.
If you have any formula left do every other bottle whole milk- that's how we transitioned my son his ped said to do it that way. If not i would just do the water thing. As long as he doesn't have a sensitive tummy it should be fine. He may not like the taste of it though so may have to get used to that
completely understand. I know many people who do it here. I live in a very secluded area that's pretty old fashioned so these types of things are just normal around here. same with home births haha