Floatable power supply one one side of isolation amplifier
You can use a DC-DC converter, like the Racom REC10-xx15DRW.

Alternatively you can use a mains powered +/- 15 V power supply, like the Racom RAC15-15DB, which can supply 2 x 500 mA at an 82 % efficiency. Isolation voltage is 4 kV AC. 

edit re your update of the question
A dual power supply has 3 connections: the +15 V, the -15 V and ground, which is what the + and - are referred to. That's your floating ground. (If the supply wouldn't have that ground it would be a single +30 V supply.)