All iText products can be downloaded from SourceForge. iText's project number at SourceForge is 15255; iTextSharp's number is 72954.
You can find more info on the jars and the DLLs and the extra jars that might be needed on the binaries page.
On the source pages, you'll find out where to find the source archives (tar.gz, zip, sources-jar) and you'll find out which classes are part of which jar/package. You can also browse the highlighted source code (with links to the API documentation).
There's also a page explaining how the SVN repository is organized.
Before you download iText (before you download anything from the internet), you should be aware that software is copyrighted and that in most cases there's a license. Don't worry: you can use iText for free, but please read the acknowledgments and copyright notices first. Especially the licenses are important: the MPL and LGPL.
