Hanger Steak [SD]
The only place where I've seen it in steady quantities is at Siesel's Market, the sister market to Iowa Meat Farms. I'm sure IMF has carries it too.
There were rumors floating around that it's been spotted at CostCo, (apparently their meat supplier sent them some but not as a regular stock item), but I've never seen it there. Bristol Farms had it when they opened, but on a subsequent visit when I asked their butcher it was only available upon request/special order. The last I checked Tip Top Meats did not carry it either.
Attached to this post is a camera phone pic of what I saw at Siesel's. On occaision if you don't find it under the counter, just ask the store help and they will retrieve it from the back.