I might have some friends that have fiber couplers for sale so I'll ask around.. but usually i work around the 300 ~400~500mw minimum so i reckon getting enough power isnt a big problem.
I thinks it is easier to work with a power level that is just below the max the OSA can handle. Typically adjustment of a fiber coupler can be VERY sensitive, making the reading jump between almost nothing and too high. This is especially the case when coupling into a single mode fiber. As said before, due to the age of your MS96A, I suspect this will be a multimode fiber input, and this makes it normally quite a bit easier (9µm diameter vs 50µm diameter make a huge difference in surface area to couple into).
You really need laser protective glasses with the highest OD rating for the wavelenghts you are working with (not a chinese gamble of glasses).
I bought my glasses from uvex (germany) and one from laserscope (USA). That have the OD rating all listed.
Good to hear you have high quality glasses, but take into account that a 20 watt invisible beam in not only dangerous for your eyes...
As an alternative way: Perhaps multiple beam splitter stages are used to reduce the beam.. gradually more splitters are added and each stage measured with a handheld power meter till a desired intensity is reached? I have a laser power meter (continuous measuring) and also a handheld power meter (interval measuring) to do such a thing.
About using splitters or other optical components, take also into account they have maximum allowed power density (power/surface area), so the wider your beam, the easier they can handle in. A beam expander could be an option to achieve this.
Good idea about using the power meter first. If you put the open end of the fiber you are coupling into just before the power meter, it will give you an accurate reading of the power coupled into the fiber (and due to the fast update, it is also way easier to adjust the coupler). Once adjusted correctly, you should be ok to attach the fiber to the MS96A