Craigslist is the way to go. Your best bet is to create an alert for the item you're looking for. I use IFTTT and create a craigslist recipe. It works very well.
I have all kinds of Sunfish parts. I am located in Rhode Island. I have new booms, new masts, new rudder assemblies, et. Of course I have used as well. I also have new sails.
If you are looking for new parts then your price will be pretty standard, most go by the prices listed on the Laser Performance website. Check to see if there is a Laser Performance Dealer nearby and they may cut you a break on shipping: http://na.laserperformance.com/dealer-locator
Contact me at cjo1023 at yahoo dot com as I have all the parts you are looking for. I have used and new spars, used and new sails. Race and recreation sails available. Located in RI. Cannot ship spars.