Up at the top of the page there is a heading entitled "advertiser's section". Those are companies that contribute to this site. You might want to check with some of them to see if they have anything or can help you.
As far as GM/OEM, there is a GM dealer that sells online at a discount - it's gm parts direct dot com. But, if it was me, I would check with the supporter's of this site, first.
Another possibility is pmd cable dot com. I know he has something special for the 6.5 - maybe he has something for the 6.6, as well.
1998 K2500 Suburban 6.5L TD 3.73 rear, Ron Schoolcraft 18:1, Kennedy ECM & IC, Timing gears, Splayed main caps, 3.5" Kennedy Exhuast/No Cat, K&N Filter, Boost/Tranny Temp/EGT(Pre Turbo), Ceramic-coated Manifolds, 195 Stat's, 265/75's (VSSB Adjusted) 7,000lbs (on a scale) Remote Mount Oil Filter, Remote Oil Pressure Sensor