Ultra-narrow beam spread LASER diode
The best light source I found is this one. It has a reasonable package (TO can) compare to other products that do have a narrower beam width but are not as convenient.


Laser diodes need collimating lenses to form a beam.  It is counterintuitive, but since lenses Fourier transform light, you actually want a diode with a broad divergence angle. You then place a lens in front of it which gives you a tightly focused beam. If you tried that narrow divergence angle multimode laser diode you linked you will be very disappointed with the beam it produces (which will resemble an LED).

However, all of this is already handled for you if you buy a laser diode module. You can get one that will focus down to hundreds of microns on Amazon or eBay for a few dollars.


  I computed a Matlab model based on some equations I found online


That page is talking about Gaussian beams, but the laser diode you have picked (and virtually any practical laser diode with a 1 degree divergence) is a multimode laser which does not produce a Gaussian beam.  The overall divergence of multimode beams is complex because they are composed of many different electromagnetic modes that each diverge at different angles.