Someone in Reddit ask the same question on a thread. This is the reply he received when he contacted AMD Support:
Dear Ryan,
Your service request : SR #{ticketno:[pmMeIfYouWantTheNumber]} has been reviewed and updated. Response and Service Request History:
Thank you for the email. I understand you lost the RGB cable and wish to purchase the new one.
I am sorry but RGB cable are bundle with wraith spire Cooler itself. AMD doesn’t have any option to provide individually. I suggest you to please check in a local market and also check for an alternative cable.
Thanks for contacting AMD. In order to update this service request, please respond, leaving the service request reference intact.
Best regards,
or
AMD Global Customer Care
So AMD can't help you get a replacement RGB Cable for your Wraith Spire.
Only way is to purchase a USED AMD Wraith LED Spire and use its cable. But they sell for about 30-40 dollars on Ebay.