I don't know where the hell to get a set of Bright plate armour suit! ..

You know when you go to the Arena? It's in there to buy. Very expensive though. Some people say that's the only place to get it but not true. Also, usually outside of the arena area (near that rock troll that comes up) are often traders.

These traders often have that bright armor you need for the demon door too. Bottom line, check all traders. They have nice stuff sometimes. Nice weapon inside the door if I remember correctly. Hope it helps.

Wandering traders sometimes have parts of it but the place to get all of it is in the Arena

The only way to get a set of bright plate armour is to buy it in the arena waiting area shop. You can also buy the chest piece and gloves from wandering traders around the witchwood lake area and you can also buy some pieces from the knothole glade general store

Thanks guys. I'll let ya know if it all works. Im WAAAAYYYY past the Arena, so I guess I'll have to look around withcwood lake and knothole glade general store...

Simple and expensive!
Talk to the wandering merchants (the ones highlighted in green), and every now and then one will be carrying a set of bright plate armour. Then you kill'em, because they are rich!

Wandering traders sometimes have parts of it but the place to get all of it is in the Arena

You can get the boots and the leggings from the blacksmith in Knothole Glade, and the gauntlets and the chest piece from wandering traders.