If you have linked your Android smartphone with your Google account, you can easily find the IMEI number of your device. I assume that every Android user has a Google account and signed in using the device.
Here is what you have to do-
1. Go to the link https://www.google.com/settings/dashboard?pli=1
2. Sign in to your Google account if not done already.
3. Click the arrow next to Android
4. You will get a list of Android devices linked to the Google account.
5. Look for your device and note down the IMEI number.