Floating supply voltage
There are several options for obtaining isolated low power supplies.

The simplest, though not necessarily the cheapest, is to buy an 'isolated DC-DC converter' from your favourite online parts supplier.

A partial isolation solution would be a pair of small capacitors, driven differentially with a high frequency, going to a full bridge rectifier. This is not a high voltage or safety isolation, but does allow the grounds to be at different DC potentials, up to the DC rating of the capacitors. The higher the drive frequency, the smaller the capacitors can be to transmit the required current. The differential drive minimises the residual drive signal that appears across the grounds.