I've always had great luck with Digikey.
If you want to follow Tpreaves advice and go for value it might be worth a little scrounging at your local Goodwill store. Mine has an entire sheet of pegboard covered with orphaned power supplies, mostly from laptops. I've almost always been able to find a well-built one with the right output. It's easy to swap out the connector for one from Radio Shack if the original one on the wall wart doesn't work.