most common is prolly aluminum. People use wood as well. Cast iron seems silly. :)

I have been building titanium staves for about a year and a half and will definitely recommend Titanium Joe. He handles small orders (you can order one piece at a time from him). He is roughly equivalent in his prices and you can always do better if you order in huge quantities elsewhere, but you need to do orders in the thousands of dollar range. The greatest thing about it though... he is just a great guy to deal with. You can talk to him on the phone and get advice about alloys and ordering, and he will always go out of his way to get you your stuff.
Having said all that, if you are buying for fire staves I will recommend 3AL-V alloys(AKA grade nine annealed). Mine have proven amazingly tough, light and the alloy is drillable, workable and weldable. (if you want to go that far)
I also have a few titanium staves here that I will be bringing to FireDrums that are for sale. A couple of 48" x 1/2", a 60" x 3/4" contact staff, and a breakdown titanium staff kit with a combination of four different staff lengths.
I will be happy to let you try any of these and give any advice I can about building your own staves at FireDrums.