The best place to get that is to farm the Admantoise in Eden. its not that hard, and even though it takes ages to get 1, you'll be glad because you should have a lot of platinum ingots by then.
theres 2 ways to go from there, but make sure you have a collectors catalogue. the reason is because even when you get 1 Trap, the cost to get ultracompact reactors is staggering, and you need these to level the weapons up. so you can either use the money to power up more weapons or buy Traps, (dont buy em). i got 7 Trap drops by the time i finished and i still didnt have enough ingots to get 1 of each weapon (and just 1 of everyones ultimate weapon) powered to their max levels.
imo, the system is flawed because theres some weapons you'll never use, and the only reason you do this quest is for the trophy, and theres no way to make money fast like in other FFs to power up the weapons straight away.
