How to get material office stuff for wendy?

If you're talking about Fragments, the best way to obtain a lot of Wendy Fragments is to purchase the VIP packs (You'll see them under the VIP tab that is on the top left corner of your screen when you're in your base)

If you're talking about officer materials to evolve wendy, the best way to obtain those materials is to clear the "Special operations" where your officers fight the tan army (This can be accessed through your Chinook).

Another way to obtain a lot of officer cases is to capture "Warehouses" which are also in your special operations page (accessed via chinook). The warehouses generate officer materials every hour 😃