It's available to buy in the shop for heroes.
Units can also wear the armor if they have plate mail proficiency. Just be ready for the massive crystal cost!
I appreciate your reply- though now I'm more confused than ever.
I just looked at my original inquiry, and I can see that I missed a few points there.
"There is no price listing for Champion's Armor" isn't strictly true. It does actually show a price- of zero. That's why I wasn't expecting to see it in the shops.
Although it isn't apparent from my original posting, I did look in my kingdom's shops for the armor- and it isn't available.
How much crystal does the armor require? While I think I have quite a bit, it IS possible that I don't have enough to offer it for sale. (I imagine that if it can be made, DEF 37 armor would require a LOT of crystal...)