Well, the first thing everyone will need to know is.... location! Where are you at? Shipping a 25 or 27 inch monitor regardless of whether it's new or used is expensive and you have a 50/50 chance of it arriving damaged. It's best if you can find one locally and drive to pick it up yourself.
Look in the Yellow Pages of the nearest large metropolitan city under the heading of "Amusement Devices". That way you can find a large coin machine distributor that has a parts department where you can buy new or used monitors.