You have a few options. Buy an oem off eBay, Facebook, or member here. Prices may vary, depending how much someone values it. The only aftermarket exhaust currently still in production (that I am aware of) is DG (rcm). They are stupid loud, unless you like that kind of mind numbing noise. A Honda 400ex head pipe can be modified to work in a 350x frame. The muffler portion from a 400ex can be used with either 350x or 400ex head pipe with some modification.
Or you can search for an old aftermarket pipe such as supertrapp, bassani, or various others.
85 Kawasaki KLT 160
85 Kawasaki KLT 110
82 ATC 70
2000 Scrambler 500