Stolen iPad

It would be either on the original box, receipt, or if the iPad was signed in to your iCloud account and Find My was turned on, you can find it on the Device list on another device signed in to your Apple ID or at the https://appleid.apple.com website under the devices section.

Link-> Find the serial number or IMEI on your iPhone, iPad, or iPod touch - Apple Support

Sign-in to your AppleID account using a web-browser:

https://appleid.apple.com

Tap the devices tab - and find you lost device in your list of devices. Here you will find information for your device - and if it is a WiFi+Cellular iPad or iPhone, will list the IMEI.