A generic installation instruction is here: How to install Discourse.PRO plugins
You need to install 2 plugins: df-core and df-paypal-buy-now.
According to the generic instruction the repository links are:
https://<your login>:<your password>@code.dmitry-fedyuk.com/discourse/df-core.git
https://<your login>:<your password>@code.dmitry-fedyuk.com/discourse/df-paypal-buy-now.git
<your login> and <your password> are your credentials for the repository code.dmitry-fedyuk.com