When I started making loofah soaps I started with PVC piping 2 1/2". The only problem with using PVC piping is it's bulky and sometimes hard to unmold. It is the cheepest way and you can purchase it at local hardware stores and have them cut it for you in 12" sections.
I now use a 3 1/4" round tube mold made for soaps. You can purchase them at www.soapandthings.com Let me know if you need any help or have questions how to use the mold.