How old are you? The best way is for you to buy it yourself. If that's not possible, the best compromise is to go halfway. You come up with half of the money for it and she comes up with the other half. It works most of the time compared to letting her get it for you.
If you mean that you're not allowed to, then there's not much I can help you with. Try to convince her why it would be good for you.
