If you made a backup of your iPhone using iTunes, the serial number and IMEI number are contained in the backup data file...you will need to open that and start working your way through it.
If you do not have a backup, your only hope is the cellular service carrier you use as they will have the IMEI number recorded for your device, that is needed so you can access their network. Contact your carrier and see if they can help you with the number.
The only way to find a lost/stolen iOS device is to have turned on Settings > iCloud > Find My and then login to your iCloud account from a computer or another device with the Find My Phone app installed, go to Find My Phone and see if it shows up. The only way it will show up is if it is turned on and connected to the internet.