On Tuesday, July 2, 2013 12:18:02 PM UTC-4, Safeman wrote:
I need to source 90 of 9/16 UNC flush headed countersunk bolts/screws
overall length 2" and 10 of the same but overall length 2.5"

It doesn't matter what type of drive they have

I don't see flat-head, but regular old Allen head cap screws, 9/16-12,
in these lengths are available at McMaster-Carr; you'll just have
to countersink with an end mill. And take out a second mortgage.