Which parts?
Unfortunately, if you need a lot of parts it would probably be better to buy a new bike. Next bikes are entry-level and other than typical stuff like replacing tires and tubes, or perhaps a saddle and chain, will cost you more than a new bike will.
If you REALLY want to dive into this, go to http://next-bike.com/contact.htm which will take you to the Dynacraft Bicycles contact page.