Finding body parts and dealing with rust

Hello, I'm new to this site. I have a 1978 Mustang II Ghia. She was my graduation present, all original. v-8 302, only 59,000 miles. Was in immaculate condition when I got it, I'm the second owner & there was only 25,000 miles when I got it.  (My aunt garage kept it, never seen rain or snow!) Well, almost 10 years later shes not doing as good. I'm afraid I didn't give her my attention soon enough & I have surface rust as well as two spots that have eaten through.  I was told I need a new fender for drivers rear and passenger front, along with a passenger door.  I can't find the parts, nor am I sure I could afford the repairs at this time.  However, I need to stop and slow this problem before its any worse.  HELP PLEASE! I've started sanding, but I don't know what to do with the holes. Is there anything I can apply to convert, stop and harden the rust?  Also I had a ziebart undercoat applied 9 years ago, but there is some rust on the underbody.  What can I have applied before having an undercoat applied again?  Any advice is very appreciated.  I'm definitely an ammature, but am willing to learn.  Quite frankly I don't have the extra cash at the moment to have the repairs done, but refuse to watch her rust away and I'd love to see her sparkle again!! (It's painful what 9 years can do!)  Thank You  

check out eastwood.com, they have alot of restoration supplies including rust preventative items and rust neutralizing stuff.