While I can’t speak for your rack in particular, most pre threaded racks are either 10-32 or 10-24 screws.
You can buy them easily on Amazon.
I work in television, and in our equipment room we have 50 racks that are pre threaded. And we use these: [Amazon link](Penn Elcom S1032/HP/WA/100 High Point Rack Screws with Washers - Set of 100 a.co/d/g13kPzw)