If you have a center tapped transformer. You can place two halfwave rectifiers (with filtering capacitors) and 3 terminal regulators, like LM7815 and LM7915 to have the positive and negative voltages that you need.
The com is the center tap of the transformers and the gnd of the capacitors and regulators.
See here: http://www.scienceprog.com/dual-polarity-power-supply-source/