Paying for open source software sounds good on paper, but if it is required, the software will never accumulate the users to make the development have any meaning.
Based on what you said, I’m not sure what you mean by “open source”, but Free Software gives you the right to distribute the program (www.gnu.org/philosophy/free-sw.en.html#four-freed…). So anyone who owns a copy can legally share it with you. There are commercial Free Software projects. The game Mindustry is one example.