There are a variety of ways to get more armament slots in Warframe. The most common way is to buy them from the Market, which can be accessed through the main menu. You can also find armament slots as random drops from enemies, or as rewards for completing missions. Another way to get armament slots is to trade with other players using the Trade function in Warframe. Finally, you can purchase certain packs that come with additional armament slots from the Warframe website.