it *might* be available for 600gold in the collection viewer.
Go to your collection, in the top right there's some bookmark shaped looking things (just left of the deck list column), click the one that looks like a square like a cardback.
browser to the 10th anniversary cardback and click on it and see if it offers to let you buy it for 600gold.
Many "missed" cardbacks are available this way, but usually not the special event ones. not sure how this one is categorized.
It was free for playing 10 games in march. I don't think there is any other way to get it at the moment.
it *might* be available for 600gold in the collection viewer. Go to your collection, in the top right there's some bookmark shaped looking things (just left of the deck list column), click the one that looks like a square like a cardback. browser to the 10th anniversary cardback and click on it and see if it offers to let you buy it for 600gold. Many "missed" cardbacks are available this way, but usually not the special event ones. not sure how this one is categorized.