Vending machines - There's a decent chance to find various types of cakes/pastries, including strawberry cake (grab any other pastries listed below as well).
Lockers - all lockers may have, but especially those in the mazon stadium/field Secret passage, as it's not uncommon to find multiple there (also check the vending machines in that secret passage!).
André Patisserie - There is a pastry shop in the Grey alley of the mall to the west of spawn (directly southwest of the Black Mages cybercafé; inside this pastry shop is a cake slice sitting on the counter (decent chance to be salvageable). Warning: shop has a lot of higher level mobs.