You don't need to pay a model, you just go to a coffee shop or anywhere else that gives you the opportunity to watch people.
Your goal: to write a quick sketch of the person or people you see.
In no more than five or ten minutes, write a short scene featuring them in any kind of situation that pops into your mind.
That older man over there looks worried--what might be going on? Maybe he just got back from the doctor's office. They did tests but the results won't be in for a couple of days. He's thinking about his life so far and what he meant to do and may never get to do if the results are a bad as he fears.Your "models" won't be sitting still for you, but that's good--often it's body language or posture or little mannerisms that tell you a lot about someone. Like that young couple. She looks totally in love with him, but when she wasn't looking he gave the waitress a flirty wink. Is he already getting bored in this new relationship, or is it just an old habit? What trouble has his flirtatiousness caused him before?
The idea is to write these without worrying about whether they'll ever be published--but I wouldn't be surprised if some of them did find their way into other parts of your writing.
At the very least they'll sharpen your perception and make those visits to Starbucks or Caffe Nero or your local mom and pop coffee shop a more creative experience.
(Another source of inspiration is my free monthly "Brainstorm & Focus" e-bulletin. You can get it by sending a request to [email protected])