I was recently given an old Cintiq 15x sans pen and cables. I'll make a separate post on cables shortly. But since it took me a few hours of searching, I thought I'd condense this and post it and hopefully save someone else some time. The 15x and the 18SX can use the same pens per all info I found. I'll be referring to the 15x as it is the only one I tested.
The Bad News:
*The Cintiq 15X pen and all compatible pens are discontinued.
*The Grapphire and Intuos pens do NOT work with the 15x.
The Good News:
*After a lot of searching. Wacom-asia has a compatibility chart here: http://www.wacom-asia.com/aptky/607/pen.htm
** In case that goes away, the pertinent info is that all PenPartner and UltraPen wacom pens should work. These are pens that begin with the model number UP-###E
** For search purposes, you want: PenPartner, UltraPen, UD Series, PL-300, PL-400, Cintiq 15x, Cintiq 18x
On ebay I wasn't able to find a 'Cintiq' pen f