Most power supplies are very specific about what they provide. You should see a label on the power supply that indicates which of the ring (outermost, in this case) or tip (innermost) is positive and negative. Makes sure that this lines up with what the empeg needs, also as indicated by a label. Also make sure that it provides 12 volts (I believe the documentation says 11-16 volts) DC power (the DC is very important). It should be rated for at least 1A (also listed as 1000mA).

After you've found one that has those specs, then you can find if the actual plug fits in the socket properly. You specifically need one with a 2.1mm center pin and a 5.5mm outside diameter.

All of this is listed in the FAQ, actually. You've probably already read it.

I was under the impression, though, that support still had some US adapters for sale. Is that no longer true?