I actually had a weirdly hard time finding 'Save Me' by Mona Kasten in English for a while. I think the title throws off searches because it's so generic. Your best bet is Amazon Kindle or maybe Apple Books; I found the official ebook there. Kobo should have it too. The English version came out a few years ago from some smaller romance imprint, so it’s not on every major site.
You might also try Scribd, as they have a lot of those translated German romance novels in their subscription. Avoid random PDF sites—I clicked on a few that were just spammy and full of pop-ups. Honestly, I ended up buying it after striking out with my library’s Overdrive, which didn't have the license. It’s a cute, angsty read, typical of her stuff, but definitely worth getting from a legit source so the author gets credit.