If you go to the dragon Lair follow the path until you fint a dragonslayer named Galanoth. Do his Quests and then buy the armor. Tip] the last one requires a large group because the red dragon has 20,000 health and needs to be beaten 8 times!
If you dragon slayer quest, then you need to finish all of Galaroth's quests. Ultimately you get the last quest which is to kill the red dragon 8 times. Then, after you turn in that question, you go back to Galaroth and he gives you access to his shop and you can buy it there.