Chuck, have you asked, if any EDM company has some leftover pieces?
Graphite is quite expensive, as I ordered two 20mm(0.787") bars long time ago. Despite that, it has unbeatable heat expansion factor, so it won't stuck on the cylinder, when heated(cylinder might expand more, than graphite piston, depending on cylinder material). Also it is totally self-lubricating material.
If you manage to get suitable size graphite rod, be sure to have a vacuum cleaner running, when machining it, to protect your lathe and other electrical devices, as graphite dust is indeed an electrically conductive material.