KAT,
You don't need to match those screws 'zakly. As long as it's a slightly larger screw, it'll self thread the hole during install. I think that those are the same ones that I replaced with pan heads in case my tires contacted them during tight turns. I picked them up at Lowes.
AFAIK, Ace buys all of their fasteners from Fastenal (at least the one local to me does) so if Fastenal doesn't carry a specific screw, then Ace prolly doesn't either.
Tom