Updated version with links can be found here:
Sanahtlig's Kamihime Project Guide: I’ve missed events. Is it possible to get the kamihime, weapons, and eidolons I’ve missed?
Event weapons (Raids)
Raid weapons were made available in the Orihalcum shop in November 2017 (DMM version) for 2 Orihalcum, limit of 1 trade-in per weapon per month. These are obtained from Guild Orders, the latest super high-difficulty content, and are also used for 4th Weapon Limit Breaks. 1-4 Orihalcum can be obtained per Guild Order, depending on player achievement, and Guild Orders are refreshed every 2 weeks. Veteran players without maxed Gatcha SSR weapons will likely need 1-2 Orihalcum per month (per element) for 4th Weapon Limit Breaks of Advent and Union weapons, and the rest should be available for exchange in the Orihalcum shop.
Because of this added feature, players might want to hold onto spare Raid weapons for any element they could see themselves using in the distant future.