Buying a vibrator is going to be tough for you since you need to be either 18 or 21 to enter a sex store. But I don't think you need to buy one because I've been to one before and most of the vibrators are ridiculously expensive.
But here's a funny story: I went to my local Walgreens to buy a few things and I was in the medical care asle. There was a personal back massager for like $10 dollars. With a kinky curiosity, I bought it and it makes a nice, cheap vibrator.
But if you're tight on money, use some stuff around your house that vibrates. I also own a Venus Vibrance razor, which is another small substitute I use.