Some of the rarer tubes will get harder or impossible to find but there are ways to adapt to others. Whenever this topic comes up the fact is raised that millions upon millions of tubes were made during their heyday and caches of them are still being discovered, so there's a likelihood that most common tubes will be readily available for a couple of lifetimes.
As to learning about radio, there's nothing like reading to understand, and experimenting. There are many ways to gain experience but as a kid I scrounged old junkers and used the parts to build sets copying schematics out of books and Popular Electronics. My uncle and I built a crystal set, not a very good one, that picked up two locals, when I was in the third grade. After that I got into working with tubes. I never could get a regen to work. Finally one day that I remember vividly it squealed into action and was picking up DX. I ran outside my shack around the yard exulting: I felt like Armstong himself!
One good way is to take a simple working set like an AA5 and go through it with a signal tracer, following the signal stage by stage. If you don't have a signal tracer you can make one quickly using a spare amplified speaker left over from a previous computer, making up a couple of probes: one for AF and one with a germanium diode for RF. One site that I have quoted frequently takes you through an AA5 and I think is helpful as it explains each stage in turn so the whole mysterious thing is not staring at you all at once. Some may want to bookmark or print out this site:
http://www.angelfire.com/electronic/...bes/AA5-1.html