Hello friends

This is my first post making a query.
If anyone had the opportunity to read my presentation, I was not born in this country, I've only been here for three years.
Buy my first bike a Honda cbr 125 r 2009.
My friend gave me a muffler from China that looks great!
but it can not be placed in my original downpipe because it does not use screws and of course it has another size of diameter.
My question is, where can I get a big bore downpipe for my bike which can adapt the new muffler?

cheers!