Use one of the free UDID sender apps from the App Store, for example UDID Tool (or search for "UDID" in the app store). If you are reading this on your iPad, just tap this icon:
When your iPad is connected by USB, iTunes shows the UDID after clicking the "Serial Number". You can simply copy it to the clipboard (press Cmd-C, there is no feedback) and paste into an email.
This Apple Support documents explains it, too: HT4061.