My business partner (an extrovert, where I'm an introvert) put it this way: if you don't ask, the answer is always "no".
I do remember in my younger days approaching someone on the bus .. in a friendly, non-creepy way. It didn't go anywhere but we did become friends for a short period. I just don't think it is okay approaching women generally in places like the bus because it is not okay to make them feel creeped out.
Streets, transportation -- these are low-filter interstices. Maximum wariness is warranted, really regardless of sex.
If you're not interested in going to the club, bar, art class -- what are you interested in? Seriously. It's tough. If your hobbies are sticking your head in the sand / introspective study (mine are), you will need to jump outside your comfort zone or else expect some degree of social resignation.