Is it V15, or V15.1?  Huge difference, unfortunately.

There are several ways to obtain V15/V15.1 (with latest updates).

Submit a Support Request to Siemens Support asking for V15 (V15.1) with latest updates so you can "upload" a S7-1200 PLC project to help a customer out.

https://sieportal.siemens.com/en-us/mysieportal/support-requests/create

Contact your local/regional Siemens Distributor and ask if one of those applications engineers has V15/V15.1 available.  They probably have an online file transfer facility and the ability to put V15 onto a private file transfer site for you.  I know my local distributor helped me out with a very old and very rare .iso image last December right before the holidays.

MechMotivation

I have a customer with no backup of the program in their PLC (S7-1200) and it was written in V15 and TIA V15 is not available for download from Siemens. Is there a workaround to be able to upload the program or is it possible to still get V15 somehow?

Hopefully your customer is not using you to get a backup of the project they would not pay the OEM to get?

Also, you can use SIMATIC Automation Tool to obtain a backup that can be downloaded to a replacement PLC should the current s7-1200 fail.  That does not require V15/V15.1 but does not allow editing or viewing the PLC logic, hardware configuration, and such.

MechMotivation

I have a customer with no backup of the program in their PLC (S7-1200) and it was written in V15 and TIA V15 is not available for download from Siemens. Is there a workaround to be able to upload the program or is it possible to still get V15 somehow?