How to get the Ads SDK for Unity 5.6.7f1

Hi. I am currently having a problem. I am using Unity version 5.6.7f1. And I want to use Ads in my game. But I can't figure out how to get the Unity Ads SDK. I tried to search on google but its all lead to using the package manager (as you know which version 5.6.7f1 don't have). As for the trying to download the SDK through the Asset Store and it was removed and I haven't purchased it before.

**In Summary : How can I get the Unity Ads SDK without upgrading my Unity version. If there is no solution then what alternative can I use without upgrading...**