I am not in the USA, but I am should be able to look
for other screwdrivers
here in the UK. I already have one quite big one, but I think it is still
way too small for this purpose.
How big is it?
For ridiculously large flat-blade screws, look at "drag link socket"s.
For just getting extreme amounts of torque, use a bit on a ratchet, rather
than on a screwdriver handle.