The VESA mount uses SAE thread 6-32 screws. 3/8" length should be sufficient, but you may want to have some 1/2" on hand depending on your mount.
P.S. - Tired of expensive rack screws? McMaster sells the correct 10-32 Pan Head screws in stainless steel for a lot less than retail. They sell Black Oxide too, but they're about the same cost as retail. In truth, ANY 10-32 pan head will work just fine!