A huge amount of new weapons (more than 150) have been already added to ERTE. Most of them are ports taken from DS3, with their unique Weapon Arts. But there are also quite a lot of completely new ones you can discover thru your journey.
The DS3 weapons can be found in two different situations: In locked DS3 chest across the open world and the different dungeons; and in the Rountable Hold new special items shop, so you can use them to create your build as soon as you can travel there. The currency for this shop are Starlight Shards. You start with ten Starlight Shards in a chest at the very beguinning in Chapel of Anticipacion, and you can gather more of them completing dungeons and exploring the map, so it is up to you to get the weapons from chests or buying from the shop.
The new weapons can be found either being dropped by enemies, inside locked chests, or in some shops inventories, like the blacksmith Ijis shop.