I think the nicest I've seen are in the Corel packages. They've got wonderful collections.
Another source is Broderbund - the images also come with some of their software, which is relatively cheap, though they have licensing restrictions. Like you can't make up graphics for download with them, or resell for commercial use. They have several ClickArt collections.
It's easiest to check at the softwear stores because you can see samples of the styles on the packages. And check for rebate deals, those are sweet.
Another place to check is at the Microsoft site, in their image gallery. No strings, they're free.
If you care to surf, here's public domain clipart [google.com] at Google. Sometimes you can come across some really neat resources.
You can also make your own using dingbat fonts.