My G25 came with a generic brandless made-in-china power adapter, 24V DC, 1.75A, so anyone with at least that much Amps and with the right plug size and right polarity will definitely work, like the one pcpaul mentions.
Other than that, from personal experience I can tell that a Logitech Momo black, with a factory power adapter of 24V, 0.75 A, works flawlessly, including FFB, with a 19V 3A power adapter from an old HP notebook.
So if you happen to have an old notebook adapter lying around you might try that first. Remember, an adapter with too much amps is no problem, but too much Volts (over 24 VDC) is. And for polarity: the + should be on the inside (which it usually is)