DC to bipolar DC converters?
What you're looking for is a split rail power supply. I've also seen these called a dual output supply, although that name also applies to supplies with 2 different positive voltage outputs.

A PC power supply is an easy option. Many have a 12v rail rated for around 500ma.

Another option is to use 2 isolated power supplies (hook them up like you would batteries).

I've seen some good options on ebay before as well:

Mean Well PD-2512