But why on earth do you think you need to make icons to make your interfaces look good? Icon driven interfaces are the bane of UI these days. Look at a screenshot of Komodo IDE or Eclipse for example. These are horrendously badly designed interfaces. It's impossible to tell what the buttons do until you hover over them and get the tooltip. I suggest you use text unless there is an icon that better represents the concept, not feel you need to represent every idea visually.
I guess it depends what context you're developing UI for - have you a lot of users who don't speak English as their first language? Is it for occasional use or for power users to use every day? Is it a web site? Is it a desktop application?
But when you really need icons, there are some good libraries out there. I think consistency is really important; Tango for instance "exists to help create a consistent graphical user interface experience for free and Open Source software." and the rather attractive icons are licensed under Creative Commons