While searching for easily obtainable fluorescent dyes I stumbled upon the Wikipedia entry for Aesculin. This is a naturally occurring substance which exhibits a blue fluorescence under ultraviolet light (360 nm). Its main source are the bark and leaves of horse chestnut trees (Aesculus hippocastanum).
To extract the Aesculin from the tree bark a larger piece of it was broken down into small chips and put in warm/hot water.
Under normal light no change will be visible, but under ultraviolet light, the water will get blue almost instantaneously.
Afterwards, the solution needs to be filtered several times.
The next step is to dry the solution until the Aesculin remains as a powder.
Actually crystallisation from a boiling solution might produce a much higher yield, because the solubility in boiling water is 74g/l, as opposed to only 1.7g/l in cold water. The resulting sesquihydrate forms white needle crystals.