How do i know what mouthpiece to buy for my alto sax ?

I'm going to this band camp for the summer but i lost my mouthpiece and i need a new one but i have no idea what kind to get. ive been playing alto sax for almost four years now and im ranked the number one sax player in my band. before the school year ended my band teacher told me i needed a new mouthpiece cuz i was too advanced for mine he said it was about 120 dollars. plz help