Due to the interest in Relic weapons in the General chat topic, I added a section on relic weapon farming to the guide. Note that the guide entry contains many hyperlinks as sources that are omitted here.
Farming
Purchasing one weapon copy requires 30 elemental regalia (Relic weapon shop). Each Ragnarok Disaster drops 2 guaranteed regalia fragments, 1 for sub-MVP, and 3 for MVP (rewards). Regalia fragments can be traded in the shop at the following rates, which reset monthly (materials shop): 5 fragments per regalia for the first 20 regalia, 10 fragments for the next 20, 15 for the next 20. Thus one copy can be farmed per 1.5 months at the lowest price tier, or one per half-month at the highest price tier (2x the effort to farm at 3x the rate).
Spawning a Ragnarok Disaster also rewards one regalia, allowing weapons to be farmed more efficiently. Spawning requires 3 of the upper-tier idea cores (Fire Ragnarok Disaster), which can be farmed from Expert Disasters. Thus to farm a full limit-break weapon in the first month, you’ll need 180 upper-tier idea cores (90 per weapon copy).