There's no "legal" way to get ahold of old RPGA scenarios (often referred to as "retired" scenarios, modules, or adventures). There are literally thousands of these things. Some exist in digital form at HQ, some exist only in hard copy form, and some don't exist at all (an angry employee purged the RPGA database, including adventure files, back in the 80s).
To my knowledge, no comprehensive list of all adventures ever released exists, even at HQ. If anyone has such a list, it'd be Robert Wiese, who used to run the Network.
A few fan sites have a decent list of Living City and Living Greyhawk adventures, but tracking them all down would literally be a lifetime quest.
--Erik