Asus told me they dont offer or sell replacements
You spoke to an idiot. Well, probably not his fault as you probably got a tier 1 tech and they have only been trained to follow a check list. They have no authority to think for themselves or use common sense.
You don't want a "replacement". You have to have one in the first place in order to replace it. And you didn't. You need to call them back and say it was missing from the package and you want a new on.

Oh no! Wait a minute!

Sorry! I just read your post again. This was an "open box" purchase. I think you are hosed and have no choice but to drive back to Fry's. If this was a notebook, you would be fine with a universal notebook power supply. But a monitor? I would not attempt it - unless I could see the original and verify the output voltage, current requirement, connector shape and very importantly, the polarity of the connector wiring.
Sorry, but road trip time.
Obvious after-the-fact hind-sight is 20/20 lesson: Always verify everything is in the box with "open box" purchases
before leaving the store.