Ok, I'm now at the mission to combine cores 5 times.

In order to combine, you need 3 of the same type. From earlier rewards, I only had enough matching cores to combine once. Where can I get more?

1. I found a single core in the black market (for 45k), which was lvl 4 (much higher than my other cores).
2. Supernova missions (which are expensive to unlock/take a lot of time) seem to give them as a possible reward for A rank, my last attempt (SS) I got a whopping 2.
3. There's a store package for 3k ether (yeah, right)

Is this just a mission that's supposed to take a rediculous amount of time to accomplish, or am I missing something?