Transistor Substitution

[O. Sharp]

Okay: I admit it, I am sometimes pig-ignorant about basic hardware 
questions.  :/

I have a couple of DEC machines which I need to replace a few components 
on, and also stock up spares of others. With the transistors and diodes, 
however, I often can't find a direct replacement - and don't know how to 
figure out what a modern substitute is.

For a 2N3009, for example, I can find basic information and a datasheet 
online easily enough - but as for choosing a functional, available 
substitute for it, I'm honestly not even sure where to begin.

Is there a basic resource for determining modern equivalents for older 
transistors and diodes? Can someone helpfully provide me with a clue 
here?  :)

Thanks!

                  -O.-