For sure! What you're looking for is "a linear regulated DC power supply" rated at your voltage of choice with at least a 3-Amp current capacity (higher is better because you can't always trust the ratings) - with heat sinks - no fans (thanks Gary!)
I can't say that 15V is going to be the sweet spot for my headphones once I try them with the forthcoming impedance match, but 15V sure great with my speakers. I don't know how you would go about discerning which voltage gives you the sound you want with your transducers of choice, without having a way to power the MG3 at different voltages (in the range 10V to 26V, per the Texas Instruments datasheet).
But once you do figure out which voltage you want, here are some examples of fixed-output linear regulated DC power supplies having at least a 3Amp capacity:
Get one of these, too:
Unfortunately, it's hard to these in fixed voltages other than 12V (or 13.8V).
You might try a switch-mode regulated power supply (as opposed to linear regulated power supply), but switch-mode power supplies can induce noise. The only way to know for sure is to test it and see if you can hear any noise in the signal. Amazon allows returns.......
Here are examples you might want to try (they've got the correct sized plug, plenty of amps, ready to go):
If you just bought these last two (and hope that they aren't noisy), you'd be able to compare them to 12V internal AA's and the 24V power supply that came with the MG3.
12, 15, 18, and 24 would be a nice spread to test. Send back anything you don't want to keep.
Or, we could sit back and let the guys on that Audiocircle thread I linked to above figure it all out. They are in pursuit of the best linear regulated power supply.