The stock nozzle has an overall length of 10mm. The threaded half is 5mm. The threads are M6x1.0 and the nozzle opening diameter is 0.4mm
This link is to pictures of the nozzle measurements with calipers.
MP Select Mini Nozzle Photos
HobbyKing & Malyan's Alibaba store have a nozzle with the exact height and thread size but the nozzle opening is 0.35mm. It is a bit expensive for a single nozzle though.
Malyan M180 - Replacement Tip 0.35
Malyan 3d printer mating 0.35mm Nozzle
3maker J-head Extruder Hotend Copper Volcano Nozzle 0.4mm for 3D Printer 1.75mm Filament
These nozzles do not have the same thread length as the stock nozzle. The threaded side is 1.17mm shorter than the original. You can get these nozzles to work by tightening the heat break/filament guide all the way to the threaded half of the nozzle. If you miss this step filament may ooze out the sides. No idea as to why the product title has “Volcano” in it. This nozzle is far too short to be an E3D Volcano nozzle.
Monoprice has said that they are hoping to have replacement parts available for purchase from their site by the end of 2016 or first quarter of 2017.
In the meantime any nozzle with M6 threads will work. You may have to make adjustments to the bed and/or heat break.