These are a special type of Lucite block, or pen blank (since folks make custom pens from them, not pipe stems). They're called Tru-stone and are available from www.exoticblanks.com. They are also sold by Woodcraft Inc. They come in 3/4 x 5" or 7/8 x 5" blocks. My understanding is rhat they have mineral inclusions in them, hence the name Tru-stone. Some are easier to turn than others, and the company tells you which ones are more difficult to turn.
I'm collaborating with several pipe repairmen to make me custom stems. By far, my highest recommendation goes out to Floyd Norwood of Norwood Pipe Repairs out in TN. Floyd is a master craftsman who can carve beautiful custom stems. I'm sure his schedule varries, but at the moment the turn around is less than a week. I won't say what he charges as that's for him to decide, but I can say his rates are very reasonable.
Essentially, I got tired of the work involved in vulcanite upkeep. My Lucite stems from Radice, Castello and Negoita are worry free. Since I use thin surgical rubbers as removable bit protectors, the hardness of Lucite is a non issue for me. I like to clench my pipes for the whole smoke.
Hope all this helps.