Dorman markets them in the "Help" line. Most parts houses carry them. You may have to buy a pack with two that includes a new plastic elbow for 99+ - it has a short arm and a long arm and replaces the cast in aluminum elbow that is part of the tensioner assembly on the 95-98s. But both should only cost about $7. If it is the elbow, you can replace it by removing the tensioner assembly. There are some other o-rings in this area that can start to leak once you disturb them, by moving the tensioner assembly. You should be able to get all the o-rings at the GM dealer for not too much $. Two are located on the ends of the metal elbows for the heater hoses where they plug into the tensioner assembly. One is located on the cast-in metal elbow for the tensioner. The plastic "Help" elbow will come with o-rings.
If you do find it is a leaking o-ring, when you have the mating pieces apart, clean them up and check the place where the o-ring seals to make sure there are no pits or surface defects that will prevent the new ring from sealing. It is a good idea to clean up those surfaces with a toothbrush size wire brush followed by steel wool. Some silicone grease will help the new o-ring slip into place without harming the sealing material.