I believe I mentioned earlier in the thread that the original recipe used ground, dried Valencia orange peel from The Denver Spice Company. I do not think they sell in small quantities, direct. I used to buy it by the pound. That is also where the ground, dried coriander came from.
I just did a quick search and all I turned up was the Rocky Mountain Spice Company. They offer orange peel granules, but it says from a bitter orange.
Orange Peel Granules
Savory Spice Shop
offers California Orange Peel. I have no idea what oranges are used.