Try
www.Hatcrafters.com
They make them to order for about the price you pay for an off-the-rack hat (easily under $100).
Looks like their #32 could be the base. Just ask them to cut the brim down to 3 or 4 inches and leave it flat.
Also their #43 looks like it could be straight up the right shape with no changes.
I guess it depends on what the top of his hat looks like.