I don't think there would be a specific "item". It'd be more just how you put it all together.
The entire bathroom floor should be a shower floor. The walls should be shower wall panels. The sink would be an all plastic/porcelain piece (no wood), and the sink faucet would be one with a shower head attachment (sprayer hose attachment). The toilet would be a cassette style (if no black tank). You would add a shower curtain to go across the door (and any shelves if you install them) for when you use the shower.