Anyone Know Where To Buy A Used Carbon Reduction?

I was struck by this (link via Dr. Sanity):

Al Gore justifies his enjoyment of a carbon-intensive lifestyle in a speech in the UK:

He said he was "carbon neutral" himself and he tried to offset any plane flight or car journey by "purchasing verifiable reductions in CO2 elsewhere".

So, where does one purchase verifiable reductions in CO2? And how much do they cost? I'd probably be in the market for a used one since new ones always cost more.

When I want to price something around, I go on eBay and look it up to get some idea what it's going for and what the major points of quality to look for are. I searched on "carbon reduction," figuring that maybe I could get an unverifiable one for cheaper. There were only two items that came up. One was a book about climate change. The other was some kind of water filter.

All kidding aside, I really do know what he means by a verifiable reduction in CO2. I just don't know where you would buy one. Do you, like, pay somebody not to drive? 'Cuz I'd sign up for that. Now that school's out, I hardly drive anywhere at all. It's such a pain to go anywhere with all four kids.