Just buy a small toroidal (or regular) transformer and the proper connector. Like Hero999 says, most likely the device requires AC input because internally it derives negative and positive voltages, so cannibalizing a wallwart may not work.
You don't say what country you're in.. have a look at tme.eu or farnell.com ... here's some suggestions below from Farnell (uk version of the site, there may be a version for your country) :
Classic transformers :
http://goo.gl/ZM7PUm (farnell link, it's miles long so i shortened it)
Toroidal transformers :
http://goo.gl/tII6vNLinks above also have transformers that have two secondary windings, for example 2 x 7v or 2x9v .. you can connect those in series to get one 14v AC or 18v AC ... job done.
ps. I wouldn't worry much about waste of power, at most you're probably looking at 5-10 watts of power being wasted, which is basically nothing. A couple of kWh a month, less than a dollar on the electricity bill if you keep it pluged in 24/7... but you could add a power switch to disconnect it completely from mains.