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You can buyy it from the girl Mina there in the far mid east of the Old Tunnels if you have the quest, persuade of at least 12 and 75 gold.
Odds or evens means you have a choice -- press all the even numbered switches down. Face the pergamen. As you do so, the switch to your left is one, next is two etc.
The gate right there, a few paces mid north.