Go to https://marketplace.thetanarena.com/thetanbox to buy Thetan Boxes. You will see 3 Hero Boxes and 1 Cosmetic Box. Select a box that you desire.
Check the Available Items in the box and click Purchase, and then Checkout
Go to your Inventory, ThetanBox and wait for your item to arrive (in 1-2 minutes).
Information about the drop rate is available on the Marketplace, so consider wisely before purchasing.