I made my own using exhaust pipe reducers from a local auto parts store ($5, and about an hour to install). It makes a big difference.And yes, this has been discussed before. A quick search would get you all the info.
I searched and couldn't find it -- maybe on the "old" server forum?
http://www.zggtr.org/index.php?topic=70.0http://www.zggtr.org/index.php?topic=712.0
I have the insert - it makes a marked reduction of noise but still lets the warm "snarl" of the ZX14 powerplant out.