how to find an old imei number

You need the phone, the box in which it was bought or the paperwork your carrier (probably) gave you. Each of these would state the IMEI.

It's possible your carrier would have a record of the IMEI number if you rang and inquired about your account details.

If you old phone is still signed into iCloud you could also sign in to manage your Apple ID and it would display there in the Devices section.

You can also see iMEI number in your iTunes backup of your iPhone.

If you do, hover your mouse over your iTunes backup.