Simplest solution would be to get a 12v AC adapter (the kind that has only a transformer in it) and build a voltage doubler into your circuit, plus probably a pair of linear regulators, to give the required +/-12v. Such simple plug-in units are getting quite rare though, as they get replaced by SMPS units.
Failing that, get a 24v supply (with an output isolated from ground) and then derive a half-rail (ie, +12 with respect to your PSUs 0v) from that using a power op-amp and ground that rail (so that what your power supply calls "0v" becomes -12v with respect to earth).
I'm sure there are also DC-DC modules available off-the-shelf that would do a direct conversion from your preferred input voltage.
I take it the current is quite low?