Dear MaryAnn,
I think it makes more sense at this point to bid a fond farewell to the old monitor if it is ADC.
The problem is that Apple's ADC monitors ran power, video and USB through one cable. It's the "power" side of the equation that requires the brick; the monitor can't get power from the wall like a regular one does. The Apple genius was wrong. Those people seem good with current gear, but are commonly weak on older equipment made before they had to use underarm deodorant daily.
Original Apple adaptors are sky-high in price:
M8661LL
I recall there being third-party adapter like Dr Bott but they are also discontinued and scarce. I looked for one about two years ago and they were already around US$100.
For $100 you can get a nice flat-panel monitor and, for $150 you can get a really nice FP monitor.
I am an advocate for keeping old technology in service and out of the landfill, but ADC has to be scored as an Apple failure in the long run.
EDIT: Forgot your monitor was probably newer than the pre-1998 equipment this forum served. You may want to look through this part of the forums where older displays are discussed:
Older Displays & Monitors
to see it there are any better workabouts.