I want to get a bike but my parents won't let me
This is a question that comes up from time to time, but in fact, a relatively small percentage of those who ride the 250R are under 18 years of age (and therefore require parental consent).
This still holds true: Their house, their rules.
It's probably not best to "just buy one". If you are not 18, you cannot insure or register a vehicle without a parental signature anyway (in most states). The best route would be to ask if you can enroll in an MSF course (most parents will say yes). After this, parents are more likely to accept the purchase of such a vehicle, since they know you are properly trained and responsible enough to take the class to begin with. If the answer is still no, see what they think about a moped (bicycle with a very small motor; not very dangerous) or a scooter; you can then trade up in a few years.
If all else fails, you can do what most people have to do: wait until you are legally and financially independent; then you can buy whatever you want.