One copy of such a weapon requires 30 regalia of the respective element.

You get 1 regalia from being the discoverer of a Ragnarok raid.
Otherwise, you'll have to trade for regalia through the shop. 3 sets of 20 per month. First set has a rate of 5 pieces of disaster of the corresponding element. Second set is 10 pieces per. Third set is 15 pieces per.

How to get pieces of disaster? You get 2 for participating in a Ragnarok raid. 1 additional piece if you vice-MVP (total of 3). 3 additional pieces if you MVP (total of 5).

Another thing to consider is that ragnarok raids weren't all implemented at once. It was one element at a time with 1-2 months in between each. Thunder should be the first in January; Dark was the latest just this past September.