Where’s the IMEI located?

Only WiFi+Cellular models of iPad have cellular radio hardware - to which the IMEI is an identifier. If you have a WiFi Only model of iPad, there won’t be an IMEI to locate.

If you can access your iPad's settings, you can find your device's IMEI (if applicable) here:

Settings > General > About > IMEI

Otherwise, if your iPad is not accessible to you - or isn't working - you can find your IMEI by signing-in to your AppleID account from any computer using a web-browser:

https://appleid.apple.com

Having signed-in, open the Devices tab. Here you will find an entry for every device associated with your AppleID; tap the related entry to see full technical details of the device - including IMEI/MEID of Cellular devices.

Additional information about finding your IMEI can be found here:

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

Assuming you have a cellular + WiFi model, that information would be at: Settings - General - About - ???