This is a missable achievement. While some merecasters are given as part of the story most are hidden or part of side quests.
In alphabetical order.
Astronomical Merecaster - The Bloom. Inside Murden Treasure Vault. In order to access the Vault you MUST ensure that Shinwise dies.
Azure Steel Merecaster - The Bloom, Shrine of Great Chila. The merecaster is lying on an altar simply perform deception and quickfingers to steal it.
Circulating Merecaster Necropolis. As you explore the Necropolis you will meet Hiro, exhaust dialogue and he will give the merecaster as a gift. I just typed random numbers into the machine until i came across him.
Electrostatic Merecaster - Necropolis tomb 5143.(very important )
Floral Merecaster Necropolis tomb 4135
Frictionless Merecaster The Bloom. Awarded during the quest into the depths. Talk to Ishen and exhaust all dialogue before he leaves he will hand you the merecaster.
Kaleidoscopic Merecaster - Miel Avest the Castoff Sanctuary. You will need to recover this from the battlefield. The sorrow will invade the Sanctuary and you will need to escape. However this merecaster is in one of the corpses to the east of your starting position. Make sure you collect as this is your only chance.
Ovoid Merecaster The Bloom - This is rewarded for completing the quest Covetous Heart's. Dracogen gives the merecaster for streaming the Cortex. Make sure you complete the quest Familiar Skies as both are tied and will bet you an additional achievement.
Tasha's Merecaster Acquired as part of the story. MATKINA will make you enter, this is the first mere you will collect.
Twisted Metal Merecaster Necropolis. Found at the entrance to the Endless Gate. Interact with the tomb to the left.
My achievement unlocked as soon as I received the final merecaster thus the achievement is for collecting the merecasters rather than entering it and completing there stories, however I really recommend entering the meres