I guess first you should decide which board you want, 75, 166, 200, 250, or the new 75C with 3 button menu system. Look for reviews of the boards on Youtube. Let us know that, and we can provide better suggestions. "Where to buy a DNA mod?" is only slightly more narrow of a question than "where to buy any mod?"
Personally I decided on the 75c board and just ordered an HCigar 75D which is a dual battery 75C board. I like the way the new menu system works and don't need more than 75 watts in a device.
Also if looking at the Therion, I would go with the Paranormal instead (better battery compartment that doesn't damage batteries). If you want a similar menu to the 75C but want something cheaper, Lost Vapes started a sister company (Modefined) and they just released the Sirius 200W and the Prism. You might look at those.
Anyway once you decide on the device we can help you find it.