Yes, Nina Hagen is really a one-of-a-kind character and she mixed classic singing with punk. Most remarkable is probably her remake of Zarah Leander.

I did a research about legal streaming of "Step across the border" but unfortunately neither couldn't find a source. I guess, it once aired at mubi, but they only play 30 independent movies at a time.

If you're able to watch DVDs, you won't regret buying a copy or maybe you could check, if they incidentally play the movie in an arthouse cinema near you.