Actually, you've got a lot from which to choose. If you want performance, there are a number of alternatives ranging from $100 - $300+ in Flowmaster, Borla, and several other brands. If you just want a cheap replacement, your closest autoparts or muffler shop can provide help.
If you need to replace your entire exhaust system, most people here would recommend that you install a 2-1/2 inch pipe. There are any number of mufflers/tips you can put on. There are differences in the way they sound, and how they perform.
Hopefully, others here can give you some specific models for your z.