Any reputable hotrod parts shop will offer a number of aftermarket dropped axles. I have always used the Super Bell dropped I-beam axle, although there are equally good axles offered from other aftermarket suppliers.
Tube axles work equally as well, and its more a matter of personal taste than tube axles performing better or worse than I-beam types.
One word of caution---If you are going to run front fenders on the car, do not get anything greater than a 4" maximum drop, or your asking for tire/fender clearance issues.
If you are running a hi-boy style car without fenders, you can go to a 5" drop safely.
The primary reason to run a dropped axle is to get the car setting lower in the front end. The original axles were made with very little drop to them, as the car designers wanted the cars to set up higher off the rutted gravel roads in the 1930's and 1940's.