Spanish Cedar likely won't be available at your big box stores.
Check your local lumber yards, the ones that sell rough lumber and/or have their own mill on site.
Otherwise, maybe Rockler or Woodcraft. Also, Amazon and eBay have offerings at very high prices per board foot.
Board foot is actually a unit of volume, not length.
L*W*Thickness 12" x 4" x 1/4" = 1 board foot.
If you buy rough lumber and mill it yourself, take every precaution to not inhale Spanish Cedar dust. It will wreck your lungs.
I don't know what I don't know.