I would check out ebay.
Since you are under 18 you must have someone over 18 with an account bid for you. There are plenty of mufflers on ebay for under 100 dollars. Sometimes you can get them for 40-50 bucks. Take the muffler to a shop and in most cases they will weld it on for around 50 bucks. What I recommend is that you do your research before you buy the muffler. Remember you get what you pay for. There are also plenty of legend parts on ebay but not much. If you went on there already then just disregard this but if you have not then ebay will be a great place to start.