How can I get birth control pills without my parents knowing?


Im a senior in highschool. My boyfriend and I just became sexualy active. I lost my virginity yesterday and we used a condom. The condom kept malfunctioning during intercource. He kept having to pull the condom out and fix it before putting it back in. I am afraid that I might get pregnant if a condom happends to break. My parents are religious and do not believe in sex before marriage. Im afraid to ask them to help me get birth control pills because I think that if I do they will force me to stop seeing my boyfriend. I know that the pill is really expensive if you dont have insurance. Is there anyway that I could get cheap birth control pills without insurance? My sister used to take birth control pills for her acne. She told me that she would get another precscription filled and that I could take her medication but I really dont think its safe to take medication prescribed to someone else. However im desperate right now and have considered it.