There are those thick black coating agents you can spray all over the underbody. Do you want to do that yourself or go to a shop to do it?
Not sure if I can contribute much to this. I've only heard that people can buy 'anti-rust' spray cans easily and just spray all over. Heard that ones made with Teflon are pretty good to use. Try searching for 'underbody coating spray' or 'anti-rust' spray and see if it leads you anywhere.
Good luck!