These screws are quite common but there is no strict standard, so it's possible the standoffs in your case use a different thread size.
It should be something along the lines of an M3x5mm screw like these.
Would these work HONBAY 100PCS M3x5 Black Zinc Round Head PC Mounting Computer Screws Computer Case Fixed Motherboard Screw https://www.amazon.com/dp/B0748BR25Y/ref=cm_sw_r_apan_i_S9RJ1MZH6JSGKV1H64CM