On purchasing your Kontakt Player product, we send an email containing your serial number which needs authorising in Native Access, before you see the library inside Kontakt.
Emails get misplaced and you might find that you need to find a past serial number.
The best place to find all of your serial numbers would be to log into your Spitfire account and then look here:
https://www.spitfireaudio.com/my-account/serial-numbers/
This page also allows you access to your order history which may be useful for your records.
If you find that the serial number you are looking for is not there, please contact us HERE with all of the relevant information.