Other than a few things like dragon scale, rappy wing, there seems to be no mag cells on the drop list? what gives? how do you get all the other cells like heart of devil or liberta kit?

A lot of the mag cells are from holiday events (Halloween and X-mas mostly).

A few of the other mag cells are from drops (as you have already mentioned) and from certain quests.

Liberta Kit comes from Black Paper Deal 2 in Episode 4 (look around for the difficulty setting needed).

Also, Heaven Striker Coat is a mag cell as is Rappy's Beak.

Fortunately because of increased rare enemy spawns here Mag Cells are frequently found during holiday seasons.
Have a look around trade thread if you're in need of any, there are usually lots of Cells for sale/trade, Sim keeps a healthy stockpile of them, hehe.

Cheers homies