swt.jar where is it?

You can go to the Java2s and below the code you can find download button, not only this jar you can find many(official) jar files from there.

Also you can find tutorials(if interesting) for swt jar from this.

I know this answer after seven years but this may be helpful to someone who is looking for this plugin.

If you have Eclipse IDE installed, you can find the jar file in eclipse/plugins/org.eclipse.swt**.jar

Now the real download page is:

https://download.eclipse.org/eclipse/downloads/drops4/R-4.10-201812060815/#SWT

At the bottom, under "SWT Binary and Source" part. You see these files:

Windows (64 bit version)             swt-4.10-win32-win32-x86_64.zip
Linux (64 bit version)               swt-4.10-gtk-linux-x86_64.zip
Linux (64 bit version for Power PC)  swt-4.10-gtk-linux-ppc64le.zip
Mac OSX (64 bit version)             swt-4.10-cocoa-macosx-x86_64.zip

You can get it under "releases" on the SWT eclipse page.