This comes not from personal experience but from a friend, but
www.tender-care.com are apparently very discreet in terms of packaging and delivery and they are about as cheap as you are likely to find. Their nappies are meant to be of a decent quality as well so they might be another option to consider, though buying in person would likely be a lot easier to hide from your parents. If you could pluck up the courage to go to a shop (or maybe twist your boyfriend's arm and get him to do it...) then it would be worth looking up any local mobility shops because the selection is likely to be better than any chemist's or supermarket.
Good luck, anyhow.