Well the chances are that if you think she's attractive someoene else does too...I say finding the right person is about numbers and it takes time...the more people you talk to, the more you're likely to find someoene really special and interesting.
You mention it took this girl 20 minutes to mention her boyfriend....big deal, it's better than you 2 getting to know one another, going on a date, having sex and then her saying something like..."my boyfriend loves it when I ______ during sex, did you like it too?"
The fact of the matter is that for most women the fact that they have a boyfriend isn't the very first thing you mention in a conversation with ANYBODY, male or female. If you want to know someeone's status before you start talking to them maybe you should try an online service where it's posted with a person's profile (and hopefully true).
I think that when you're looking for a SO you try and read that into everyone elses actions and of course you're more sensitive to the fact that everyone else seems to have a SO...I think that convo above would just means that she thinks you're and interesting person and this isn't a complete waste of your time. Since she's taken I'd ask her if she has and single friends because one of the best way to meet people is through friends.
So maybe if I were you I'd stop the one track approach to finding a GF and maybe just focus on meeting new people and new friends...up your odds of meeting the right person and you will.