Dishwashing liquid formulations have changed over time, making it difficult to find one which contains sodium stearate. This experiment can still be completed with the assumption that the dish soap contains a theoretical sodium stearate concentration. It will work regardless, with the difference that in the end, the molecule's width estimate will not be accurate for sodium stearate.

You should just make sure that you use hand dishwashing soap rather than soap intended for automatic dishwashers. Examples of brands you can use are Dawn, Joy, and Sunlight.