Birth control

I'm 15 going on 16, and I have found myself in a very awkward situation. I am dating my best friend's ex boyfriend, and me and him are hoping to move forward in our relationship. We have a date in mind, which is a day or two before my period and we are going to use a condom. The problem is I cannot openly tell my parents and cannot afford birth control pills. We want to be safe, and I want to make sure nothing beyond our control happens. Also I cannot buy emergency contraception because $40 isn't something I just have on hand. I would really like advice on the contraceptive rather than being told off about having sex to young. I am unable to use female condoms and neither of us can use latex. Does anyone have any ideas of what I could do to prevent pregnancy?