Try a hardware store.
Not a place full of small household dry goods like mops & pails and canning supplies and dog collars, but a hardware store.
Bring your holster and the screws you want to replace. Show them to the guy in charge of the screws and fasteners department. If he does not do it automatically, ask him to guage the pitch and thread of the screws you brought in. Then let him show you what he carries - you may want to consider alternatives to flat-head, or you may be insistent because you do not want to buy yet another driver.