Put a shoe on your head.
I mean, what the above guy said, from the box rewards from the level 50 daily board quests in Candeo Marsh.
However, specific boxes only give specific kinds of gold weapons, e.g. max's gift (forgot what quest it's from) only gives max's magical pistols or w/e (gold guns). Mina's gift only gives golf staff, so on, so you'll only see lots of gold weapons on marketplace on specific days.