Have you looked for Cedar at Lows and Home Depot?
I have not found good clear cedar at the real lumber yards here in Ft. Worth, its all the same as the big box stores.

If you are willing to go back again and again, you can usually find mostly clear boards. I usually only find one to two per visit, and i look thru as much of the wood as I can.
Try the bigger boards first 2x as big as they have. If you find2 boards 2x12x8' that is just about enough for a Great Auk.
Persistence counts here, and you can't be too picky or you will never have a chance.

The wastage is high, due to cutting around a knot, but still cheaper than ordering pre cut strips.

Actually I wanted to try Basswood, but the yard ran out of big long pieces, and now doesn't know where to get any more

Not the best situation, but you can do it. You'll have to get use to butt joints made on the stripping tool, make them between the forms.