We are a big fan of Grainger.
www.grainger.com
In fact, we have a rep that set us up with an account so we get a discount, don't pay shipping, and have it setup with a bar-code ordering system.
They provide you with the scanner and labels and when you need to order stuff you simply scan it and it orders it.
You can also input quantities of an item and have it setup so when you are critically low on a part it will automatically order it for your team.