Where to buy a good quality wall wart

Here is the address: http://www.rock-grotto.co.uk/pinkiepsu.htm

Look at companies that provide hospital grade electrical equipment. The requirements hospital's have are pretty tough. I have some hospital grade isolation boxes and they are great, with a low noise floor.

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.