A few of us have tried walk-in 3D/4D scans before, and from what we've gathered, Singapore Diagnostics is a good option, they have quite a few centres around the island and do accept walk-ins, so you don't necessarily need to book in advance. The scans are really lovely because they focus on your baby's features rather than the internal stuff, so you actually get to see little expressions like yawns and smiles in real time, which is just the sweetest thing. One of my mum friends also mentioned that some standalone baby scan studios offer this service too, so it's worth asking around your area to see what's closest to you!