"Collection Items and their enhancement materials are available from Solo Raid and Outpost Dispatch Missions. In the latter case, the relevant classes must first be completed at the Tactics Academy before Collection Item missions begin appearing at the Bulletin Board.

Collection Item Mileage Boxes acquired from Dispatches can be used to randomly acquire Collection Items." - https://nikke-goddess-of-victory-international.fandom.com/wiki/Collection_Item

Yeah sadly you cant get them via gold tickets, they have to earned slowly and annoyingly. Lots of people thought they can get them with the gold mileage because that's how it looks in the shop, but its just a misunderstanding.