hey chino!
what do you mean by
core stems?
Ive never heard that term before,
I think you mean something like
zero offset stems?
Anyway, you can always get the dragonfly Mike S bar/stem combo;
http://www.dragonfly.de/Fishbone made a
straight up stem, there's one on e-bay right now;
other than that I dont know of any more.
GT had a prototype
straight up stem on the prototype gt show back then but I think the production gt show came with a gt piston stem.
You have tons of other stems with short reach from 26mm and up from:
KHE, BIZHOUSE, BRICKHOUSE, KGB, QUAMEN, HARO, etc..
Hope this helps you.