Shelby owners seem to be a pretty tight-knit bunch, and many times a real Shelby will stay within that inner circle of ownership. I.e., a Shelby will typically be purchased by another Shelby owner before the car is ever made available to the general public.
That being said, I'd hit up the Shelby registries, then start talking to people that actually own them. Maybe one of them is willing to sell their car. If not, maybe they know where to look.
In any case, a 4-speed Hertz is going to be a tough find, as only the earlier cars were 4-speeds. | 
1969 4 spd Hertz Mustang 
