Is it possible to purchase or download Return to Zork?
gog.com now has a packaged copy that runs really well, and it is cheap too, $5.99.
You can play Zork I online here:
There are free downloads for Zork I, II, III & The Undiscovered here for Win/Mac (no Z-interpreter required):
Also, the original mainframe version of Zork is available. It still runs on Windows/Mac/Linux, but the content is different from the packaged version of Zork I, II & III. The site describes it as equivalent to Zork I + about half of Zork II + the endgame of Zork III:
EDIT Oh sorry, I just re-read your question and realised you were specifically about Return to Zork. My apologies. I'll leave this here though in case a Zork enthusiast finds it useful.
eBay is probably your best bet.