How can a PhD student find a collaborator who might assist with programming in exchange for co-authorship?
The best solution would be finding someone locally. Contact the professors who teach advanced programming-oriented courses in your university (and also in Berlin, for instance, since the two cities are close) to see if they have someone to suggest among their past or present students.

Finding someone to do the work remotely seems more risky. It is difficult to train and mentor someone without meeting in person at least a few times.

And, in any case, when hiring a student for such a job, keep in mind that  there is always the risk that in the end this move will save you a negative amount of time, if you find the wrong person.