You can flirt with a large variety of people and the social consequences are pretty low. Thinking about how people respond to it is also free. If it's rejection you fear, say hi to strangers. Some of them will ignore you (or maybe even be bothered). That's rejection. It feels like rejection. But the stakes are so low it's safe and you'll become inoculated to a degree or at least comfortable with it. If you want to get better at asking people out, either roleplay with a friend or ask people out that you find interesting, even (especially) if you don't have a crush on them. Bonus: You might get practice at dating, or just find someone you really like.