mostly they wont actually snort they will blow, or just act it out and have a cut away, but for the powder itself lots of things can be used - i use canderel - but stuff like flour, icing sugar, talc, chalk dust etc can be used - if they have to dab and taste it, then obviously stuff like talc is no good
if they were to film a real closeup and the actor actually had to snort something, you could use the canderel as it is edible so wont do any harm but you could also use certain medical nasal antibiotic powder if absolutely necessary, though this is not ideal.
others may have found better substitutes though.