If I remember correctly, The Serpent Master had the key. (Guy in room with two elevators) Had you talked to him first and answered his question "No" he woud have given you a key. How you are supposed to get it without knowing this piece of important information ahead of time is beyond me!
You might want to do a search on Serpent Temple threads....or threads on Keys...someone may have posted info on how to edit it into your inventory.
Hopefully, the method mentioned in the above post will help you get it without having to re-load an older game or edit your files!
Good Luck!
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