macOS server 5.4

How to get macOS server 5.4 today ?

I have to install macOS server for HighSierra 10.13.6, in order to have a slave of OpenDirectory.

I can't update my MacPro to 10.14 now because of a specific software that doesn't work under Mojave.

So I 'm loocking for à 10.13 release of macOS server application.

Can someone help me? knowing that i already bought macOS server 5.3.1 for Sierra, I can legaly download this update (regarding to Apple policy).