There are several sources here under the builder's directory. If you're building a boat the most cost effective way to do it is buy northern white cedar rib blanks and substitute western red cedar, which you should be able to source locally, for the planking. Shipping is what drives the cost up especially on the longer planking stock. The only real drawback to using WRC for planking is it tends to split more than NWC but predrilling for tacks in areas of sharp bends usually takes care of that issue.
The 60" rib blanks are obviously more expensive than rough cut lumber but a bundle of rib blanks can ship UPS. Go to the Builder's Directory and click on Suppliers.
If you have the ability to mill your own lumber, I'd recommend Whitecedar.com. I have used them several times in the past, and have been generally happy with the quality of the rough lumber I have received.
In restoring my 1911 OTCA I was able to find some acceptable boards for planking by sorting through the Western Red Cedar fence pickets at the local Home Depot. I had to go through quite a lot but was able to find some acceptable boards that I could resaw for the planking. It was about $2.00 per picket.