This information is readily available here
It appears they have two recommended suppliers in the USA for the 50-70 mesh glass beads and one for 40-50.
When I inquired about using the smaller beads IIRC they mentioned they may break down and pass through the separator more quickly. I am still on the same initial tub of 50-70 media I bought from Grainger.
I had similar concerns with the machine drowning me in consumables but this has not happened. The blast has been absolutely indispensable I cant imagine running SLS without one or similar.