Berwen is in the center of Tel Mora, northeast of the docks if you arrived by ship. Unfortunately, the retort is on the shelf in front of Berwen, so you'll have to be sneaky. The easiest way is to go upstairs, jump over the boxes, jump up on the table in the far corner, take one or two steps into the corner, turn to the right until you can see the shelf below, get into sneak mode and use a telekinesis spell (one with a 25 foot range works well) to grab the retort. Another way is to exploit the game mechanics by jumping onto the shelf, then immediately jump again so you are stuck in the ceiling (you have perfect sneak when you are in the ceiling), then sneak and grab the retort and anything else within your reach that you may want.
If you have already killed the Corprus Stalker upstairs, you could just buy it for an exorbitant amount of money if you don't want to risk getting caught, but it's cheaper to kill Berwen (you'll get a bounty around 40), take the retort and anything else that seems useful to you, then go remove the price on your head with Rissinia when you return to Sadrith Mora. Speak with Big Helende to finish the quest.
Another way of acquiring a Grandmaster's Retort is to go to the Services Tower in Tel Vos. Right in front of Andil, there is a shelf with a Grandmaster's Retort. You can get over by the door leading outside (around the corner where no one can see you) and use telekinesis to steal it. You may want to wait for the guard to walk out of the room, though his presence shouldn't be a problem.
Daynali Dren in Tel Mora (Lower Tower) has complete sets of Grandmaster's alchemy equipment.
Abelle Chriditte, who resides in the Propylon Chamber of Valenvaryon (far north-northeast of Maar Gan and far east of Khuul, near the coast), has two full sets of Grandmaster's alchemy equipment for sale, and one of these sets is lying on the table and floor to the left of the room.
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