You first need to know what the volts are that the scanner uses, it will more than likely be on the label or near the socket, plus you need to know what polarity that the plug should be positive to centre or negative to centre. You can then decide if you want to spend money on a volts adapter also have a look at the current it uses probably about 1.5Amp
Low power supply plug, Maplin are a source for these, they come in different sizes, or you can buy power supplies with a range of them attached. If you have a Maplin shop nearby you can have a look at the catalogue.
These plugs alone come in sizes 1.3 mm, 2.1 mm, 2.5 mm, 3.1 mm, 3.5 mm, Pages 208 and 209 of the catalogue.
Power supplies in various voltages and currents with those plugs are on pages 505 to 512.
the only thing I can suggest if you cannot find any identification on the scanner, is to buy an adapter that as at least 12 volts it may be double this, one with many different plugs centre plug set to positive and move the power up starting at about 5volts but what about twain drivers also, if you do not know what the scanner is then there?s no way you are going to get it working without the driver disc. it would not cost much more to buy a cheap flatbed scanner click here