The Super Rare/Legendary weapons can be found very rarely throughout Raid Mode, or in the case of the Pale Rider, rarely at the shop as well. Your best bet to get these is to repeat missions 19 or 20 on Abyss difficulty. If you unlocked these using the Capcom present codes, you will also receive the achievement.
The weapons are as follows:
Muramasa Rifle (Looks kind of like a gold and black, mordernized flintlock rifle)
Pale Rider (Really big Magnum)
Drake (Double Barrel shotgun covered in Dragon scales)