Argent Tournament Badges.
Champion's Seal is a reward from completing quests as a champion of the Argent Tournament. It is used as currency to purchase rewards from factional vendors with whom players have earned the right to champion. There are a number of daily quests that award seals.
Here you can buy Champions Seal for EU or US Servers.
You'll get guide for getting Champions Seal
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You can see the full list of all offers on the main page - The War Within WOW.