It's an odd suggestion, I know, but the stiffest stem I ever used (by far) was the SystemSix stem that Control Tech made for the Cannondale SystemSix bikes. Carbon wrapped aluminum, reasonably light, and roughly the diameter of my wrist. You can find them on eBay for cheap.

http://www.ebay.com/itm/Cannondale-S...item257787e51e

Here's a 120mm. They made up to 140mm, I believe.

I still use an old one on my MTB.