9V is fine, the voltage regulator inside drops anything from 7V-18V down to 5V for the system. The amperage is what you should look for, 850ma seems to be the standard for the 3rd party and it is plenty to power the SNES/SFC. Here is one I know works fine.

http://cgi.ebay.com/AC-DC-POWER-ADAP...item2eb11a3ac3