This is a folk-whistle or penny-whistle, and won't even cost a penny to download.
Versions included are sopranino in G, soprano in D, alto in G, and tenor in D (all versions are one octave higher than the corresponding singing voice).
The instruments are given as pieces. In ABS plastic they can be welded together using acetone. I have found this method to very reliably produce a functional and well sounding whistle.
You have a choice of four sizes of instrument: sopranino in G, soprano in D, alto in G, or tenor in D (not pictured). These have ranges one octave above the corresponding singing voice.
Each zip-file contains the instrument segmented into 3, 4, or 5 pieces. Choose a segmentation with pieces small enough for your printer to print, and that it can print without the pieces falling over while being printed.
When printing, the wall thickness and/or fill percentage should be quite high. A stiff wall is necessary for the instrument to sound loudly and without needing excessive air flow. If you can feel the wall vibrating as you play, that is energy being lost.
Having printed the pieces, they need to be connected together. In ABS plastic this is easily achieved by dipping the ends of pieces you wish to join in acetone for 15 or so seconds, then holding them firmly together. The model includes guide prongs that will ensure the correct orientation of the pieces. Use appropriate safety equipment when using acetone (gloves, goggles, good ventilation, etc)!
I am not sure if a similar solvent is available for PLA plastic. If not I would suggest trying cyanoacrylate glue in gel form, or epoxy. The join must have absolutely no leaks.
See the file folk-flute-and-whistle-fingering.pdf for fingering system.
These instruments were designed using Demakein, version 0.14. You could use Demakein to create whistles in different keys. Demakein can also be used to design a variety of other woodwind instruments.