There are two elbows total, but only one lower, the upper coolant elbow is plastic and goes from the lower intake manifold to tensioner, your lower elbow is built into the tensioner, and its elbow/nipple is aluminum.
If you are changing the upper elbow as well, just get the aluminum coolant elbow kit that Debra mentioned, do NOT go with the Dorman Help Kit Plastic Elbows, or o-rings from the Dorman plastic Help Kit.
You could take the o-ring off of the big end of the lower elbow that comes with the Dorman ALuminum Elbow Kit, it will fit your aluminum nipple coming from the tensioner, but if you want the recommended o-ring that was originally on that lower/elbow aluminum nipple, then it is GM Part# 24502846
They are (SEAL-0.799" ID,0.1.03 X-SECT DIA)
You can buy the o-ring from Nalley Buick Parts online, I'll try to find out the AC Delco part number for them, that way you can get them cheaper, and possibly from a local auto parts shop.
SEAL. Engine Coolant By - PASSENGER Engine Fan Coolant. # 24502846
Also, attaching a pic so you can see about where the lower elbow comes from the tensioner, its o ring is shown as 337
NOTE: Be careful when removing it, if you crack the aluminum nipple you will have to buy a new tensioner.