There are two stores where you can buy decks. Check out The Game Crafter for cheaper production, or Make Playing Cards for high quality bulk orders.
Buy online at TheGameCrafter.com
Available for under $15 and made in the USA, The Game Crafter provide an affordable way to buy Threat Agents and for developers based in the US, a quicker shipping time.
You can purchase copies from MakePlayingCards.com. This is a Chinese company that manufactures a massive amount of games. They did a great job of my prototype and I bought my first few production decks from them. I now have a store setup where you can buy the game from them. Please be aware that they are expensive for single decks and shipping costs can be quite high. Delivery is fairly slow as they are made to order and shipped from China. This is because they are setup more for mass production, shipping to warehouses.