Where can I find part time jobs as software developer

Most job search sites have a part time filter. I don't remember seeing very many advertised though. I guess that makes sense since many places want full time people to work extra hours and always be connected - there's not a lot of benefit to being part time if they expect off hours support.

If you're currently at a company, you can ask about transitioning to a part time role. Some companies are open to this, especially if you don't require benefits.

It's also possible you can find work creating websites for local companies.

Do you mean contracting? Assuming you have proper work permit, there's a chance you can't do part time or contracting in certain countries, especially in Germany if it's your primary occupation as well. You'd need permission from your employer.

If you narrow it down to what you specially you're looking for, it's going to be easier to provide you the answers.

There are some contract jobs here: https://www.arbeitnow.com/contract-jobs

I like you missed the timeline this year, but next year "Google Summer of Code"