treehugger_rss had an Earth Day roundup post here that linked to the Union of Concerned Scientists article on Ten Steps to Reduce Your Global Warming Impact. Step 2 was "Choose clean power" and it included a link to http://www.green-e.org/ - Green - e: Renewable Energy Certification Program. They certify a variety of organizations offering electrical power made from renewable resources. One way to buy renewable energy is via "Tradable Renewable Certificates."
Basically you buy these certificates as a way of supporting alternative energy and "offsetting" your own use of fossil-fuel generated electricity. (In some areas, you can sign up to receive power directly from alternative energy power companies, but in California? Bwahahahahaha.)
This page on Green-e provides a list of links to companies in the US selling TRCs. I bought mine from Bonneville Environmental Foundation at https://www.greentagsusa.org/ because they have a good calculator where you can figure out your energy use and they're a non-profit.
Green-e has a link to http://www.terrachoice.com/ which they say is a similar program for Canada, but it seems to be an "environmental marketing" company. I'm pretty sure
treehugger_rss has reported on programs like this for Canada and other countries. 