iTextSharp on GitHub
On GitHub, you can find the source code for all open-source iTextSharp components. You can download zip files with all the necessary DLLs from the iTextSharp release page:
itextsharp-all-x.y.z.zip contains zip files with the core DLL, the PDF/A DLL and the iText xtra DLL. It also includes ZIP files with the source code.
itextsharp.xmlworker-all-x.y.z.zip contains zip file with the XML Worker DLL and source code.
Always make sure that you're using DLLs with the same version number, e.g. don't mix iTextSharp 5.4.0 with XML Worker 5.5.7.