I'm playing White 2 so I didn't get it on on the Plasma Frigate. I know you can find them in Join Avenue but I only seem to get Hard Stones from the shop. As for King's Rock, Serebii says you get one at the Fan club. I'm using my own pokemon which are all above lv 90 and he still doesn't give me the item.
King's Rock is post-game in BW2. Magmarizer is only available via Join Avenue (not counting the "free" one in Black 2). You need to get a Join Avenue NPC from Black 2 who opens an Antique Shop. Their "Box" items randomly have a Magmarizer in them. --- "Murder of the living is tragic, but murder of the idea is unforgivable." - Janus, speaker of the synod
As for the other item, all I need to ado is add someone from black 2 and have them open up an Antique shop, correct?
Breeder Amando and Hiker Julio will randomly appear in your Join Avenue. They count as Black 2 NPCs, so getting either of them will help.
Incidentally, Ranger Atillo will open an Antique Shop that can randomly sell King's Rocks, and KR can be bought on certain days after you progress through White Treehollow. --- "Murder of the living is tragic, but murder of the idea is unforgivable." - Janus, speaker of the synod