Either go to a real lumber yard and look for Aye grade cedar, (Note, box stores are NOT lumber yards)
or
Do go to a box store and look for 2x stock in the largest sizes you can find.
The longer and wider sizes usually come from the best parts of the tree.
And then "high grade" it by cutting stock around the few knots that might be there.
You do not need full length stock.
Dan
"I live here in middle of the US. Any and all cedar is full of knots and will not word for building a strip canoe."