Firstly, where do I find a table of the possible '$' values?

I have found $INSTANCE returns a value which is useful to me, but not so much to a user, so I wish to translate this value to a name.  $INSTANCE returns .2 which in this case is a radio site number.  

The radio site name I can retrieve from an additional OID in conjunction with the result of $INSTANCE (the OID gives me the list of site names and the .2 returns the second site name), but I don't know what the format is to generate it in the Performance Monitor message field

So, secondly, how do I change my alert message to give the site name by using the second OID and $INSTANCE?