If you check your vault you've probably got several Warmind Cell weapons already. The only two exceptions are the IKELOS shotgun and hand cannon, both of which can only be found in the Prophecy dungeon currently. The good news is that most of these are now in the world loot drop pool, meaning you'll pick them up just running around on patrol, doing Strike missions etc. There are two sets of legendary weapons that have the intrinsic perk that activates Warmind Cell mods: The IKELOS weapons and the Seventh Seraph weapons.
Also, the associated weapons seemed a little underwhelming-before people realised the Cells themselves were OP. As I wrote in my end of year wrap-up, the only reason the Warmind Cells haven't become completely ubiquitous is that they're poorly explained in-game. You can also spec Cells so that exploding (or collecting) them will recharge your abilities, heal your whole team, suppress enemies, and more. The really crazy thing is that we're just talking about their vanilla form. In fact, they're so strong, and the AoE so wide, that stuff off-screen often dies too. Shoot one and pretty much everything on-screen dies.
Warmind Cells look like glowing orange beach balls but function like an amped up version of the traditional videogame red barrel. They're so strong, and the AoE is so wide, that stuff off-screen often dies too.