AFAIK it must be prescribed by a dr, not over the counter.
as for contraception, it's a two way street. you both need to grow up and act responsibly. you cannot demand that she take the medication. this is really a case of too little too late. if she doesn't decide to take the pill, all you can do is hope and pray (if you are a praying man) that nothing comes of it. and maybe learn from it?
also, there are only about 4-5 days/month that a woman is her most fertile and most likely to get pregnant, this usually is about 1.2-2 weeks after her last menstruation. so, although there is a slight possibility for her to get pregnant at other times, it is unlikely.
so, maybe the question you should be asking her is: when did you last have your period? ....don't know if she'll answer it or not though.