Reed and Bamboo pens are not expensive and can be found online relatively easily.However, having a bility to cut your own is an art in itself and is mastered with years of practice and repetition.
Bamboo poles can be purchased in various gardening and hardware stores, with the current COVID 19 restriction you could buy yours online. Ideally you would like to purchase a stem that is flexible and not too stubborn or too old, as the last thing you would want is to apply a little pressure and have the tip breaking off.
A flexible stem will provide the smooth strokes you are looking for. Also, the carving process of the pen becomes a lot easier if the stem is a little flexible. If you inevitably end up with bamboo that is old and hard, leave it in a bowl of water overnight and the bamboo will soften up.
Following are the instructions to create your own reed/bamboo stem pen: