Is there a way to get more money, besides finding some in the apartment, before I'm able to make money with photos? I have to buy the battery for Amy to progress her story, but only got 10$ left. I already bought the topaz for Serena, had a dinner date with Claire and bought the magical crystal required to beat the dealer in the gym.

You should already be at a point where you can do some photoshoots and get money. Talk to Katie on Sunday evening in her bedroom to start.

I found out I can do it per chance later. Thanks for answering neverless. It's a great game, I really have fun playing it :D
Random question: when I'm looking into the Character Profiles, there's a locked portrait between Claire and Katie. Did I miss something out, since the rest of the first page is completely filled?