I know it is called "PINE"wood derby but do you have to use pine in your chassis?Can you use a lighter wood like balsa? or do you have to have pine, and if so what kind of pine do most use?
You can mix it up with a combination of pine and balsa. Use balsa as your fill in or ribs. I think if you only used balsa, you would run into issues with your axles not staying secured or probably cracking or breaking the wood easily. I would imagine that you would run into trouble with weight placement as the car would be so lite that it would turn out to be a wheelie machine if you used the standard weight placement. I'm sure the pros know and can lead you in the right direction.
You can always purchase here - http://www.derbydad4hire.com/Premium-Sugar-Pine-Wood-Block-SSG1-2.htm
I prefer basswood over pine. I don't think any of the leagues require Pine, but check your scout rules before building a scout car. Many scout rules require using the provided pine block.
Balsa is too soft to use as a frame/chassis. Though, as EJ mentioned, Basswood/Pine + Balsa combos are common.