He he, thanks Mike, I do know how to sniff em out.
I did a little research, and I am pretty sure I found the right replacement o-ring from Nalley Buick Parts for $2.18 plus shipping.
This link should work, if not let me know.
ENGINE ASM-3.8L V6 PART 3 FRONT COVER AND COOLING. Fits: 2000 Buick Lesabre CUSTOM | Nalley Buick GMC Brunswick
Larger O-Ring is listed in the diagram as #343 GM 24502846
The other O-Ring on the lower coolant elbow is listed on the diagram as #343 , it is listed as GM 25537068, and they have another number listed which is 1534066, which just happens to be the part number I posted for the upper coolant elbow o-rings, so it appears they are the same after all.
I cross referenced the GM 25537068 upper coolant elbow o-rings to see if they were also the Delco part# 1534066 like I thought, and sure enough they are.
There was no alternate number listed for the larger o-ring.