IMEI / Serial # for lost iPhone-5

You can't. The only potential possibilities to find them now would be:

• The phone itself.
• The box that it came in.
• The purchase receipt you got with it when you bought it.

Ref: Find the serial number of your Apple product - Apple Support

If you do have, at least, the last two. You can provide it to the airline so that they can compare it to the serial# printed on the back of your iPhone.

*update* If you signed into your iPhone with your Apple ID AND you enabled "Find My," you should be able to gain access to this information from your Apple ID device list.

Ref: Manage your Apple ID - Apple

Either you never enabled "Find My" or someone who has access to your Apple ID credentials disabled it. Regardless, without it being enable, there is nothing you can do to track this phone.

I'm assuming that you have already spoken with the airline? If they have it, then obviously you need to work with them to retrieve it. If they claim that they don't, then it is either lost, or unfortunately possible stolen. Again without "Find My" enabled, you are out of luck ... and, if this be the case, you need to change your Apple ID password immediately.

Give the information to the airline.