Just talk to them nicely (like actually properly, have a real conversation about something else intelligently and then bring it up that if YOU earn't the money then could you buy one.
Be nice to them, don't throw a strop, do things even without asking, when they say no over and over again don't go off the handle (my sisters mistake all the time), just keep being good for them and not bringing it up all the time and you might get there.