trainers?

1) There are several Mercantile trainers. They are:
- Foroch >> Gottshaw Inn west of Kvatch;
- Mach-Na >> Mach-Na Books store in Cheydinhal;
- Khafiz >> Dunbarrow Cove (with Thieves Den official plug-in only);
- Margarte >> East of Leyawiin's Great Chapel of Zenithar;
- Seed-Neeus >> Northern Goods and Trade store near Chorrol's South Gate;
- Palonirya >> Divine Elegance in the Imperial City Market District.

2) Daedric Armour is very rare at the start, but you can find it quite often when you reach level 20 (minimum level required). There are a few videos available in youtube that will show you ways to get it much sooner, but trust me, it's very complicated and not worth the time.

Hope I've helped, good luck ;)

Traders usually train in mercantile, deadric armor can be foundafter lv. 20

Obtaining the armor as a drop basically begins around level 46 for regular pieces and Level 47 for enchanted pieces. These appear as random drops from within treasure chests and as loot from both Revered and Legendary Dragons, but these drops are very rare...

An alternative source for obtaining a full set of the armor can be found through the Smithing Skill, since pieces of Daedric Armor can be forged by the player using either the bog standard Blacksmith Forge or the special forges that are situated in special places

Examples of the special forges include the one in the basement of the College at Winterhold and there may be others that I am not aware of... But that one for sure, and of course you can use the forge at Blacksmith shops as well to a somewhat lesser effect.

This special armor is created using what are basically rare resources -- which makes sense considering that it is a rare armor, right? Anyway, you will use ebony, the hearts of Daedra that you have collected as loot, and charged Black Soul Gems combined with Dynamo Cores (the latter being found in Dwemer ruins pretty much throughout the land they being obtained from the bodies of Dwarven Centurions).

The Mercantile skill gives the player better chances of selling items for higher than their average value, and it can be trained at the Apprentice-Level from Foroch at the Gottshaw Inn to the south-west of Kvatch or Mach-Na at Mach-Na's Books in Cheydinhal. For the Journeyman-Level see Margarte at the Great Chapel of Zenithar in Leyawiin or Seed-Neeus who is at the Northern Goods and Trade in Chorrol. Finally there is the Expert-Level from Khafiz in Dunbarrow Cove and Master-Level from Palonirya who can be found at Divine Elegance in the Market District of the Imperial City...

Bear in mind though that for Expert-Level you will need to have 10,000 gold on hand to initiate training, and with respect to the Master-Level that is part of the Thieves Den plugin, just saying...