-12v power source for op-amp
In general a virtual ground would do  the trick, and then you should use something like this:
 

In this specific case however it may not be a good idea, since the ground is also connected to your oscilloscope, and to the ground of the device you are measuring on. So I would advice you to create the -12V using a simple DC/DC converter. The 'MULTICOMP  MCE12S05S  Isolated Board Mount DC/DC Converter' from Farnell is only €4,- and would work fine in your application.