Another option is to use an isolated DC/DC converter, as discussed in e.g. this thread. These aren’t that cheap, but are fairly standardized, and are available from multiple manufacturers and for different source voltages and output currents. I use tiny ones to get ±12 V from USB, mostly for testing but also for stand-alone boxes. Powerbanks are everywhere.
EDIT: But as @analogoutput points out below, you might not need this level of fanciness for a sequencer.