If your negative supply requires only a few milliamps (about 10 mA or so) it is usually better to have a single transformer power both positive regulators, and then take a charge pump to generate the negative one.
The charge pump can be built with a 555, or even better with the ICL7662. These two are ubiquitous on the internet, there are dozens and dozens of examples.
If you require higher performance, you can use one of the best devices from Linear or Maxim.