It is a plastic elbow that is known for failing on the Series 2 supercharged engine, which is what you have in the '96 Ultra. Those items can be found in the HELP! section of most auto parts stores. I am unsure of the complete replacement process, but I believe it is somewhat involved, especially if you have not done it before. Using stop leak is one of the worst things you can do, as it gums up everything, and this leak will only get worse. Also, being a supercharged engine, and not the NA Series 2, it does not suffer from the UIM failures that you would have had to deal with in the LeSabre.
Suggestion: Feed it coolant/water until you can fix it, or it completely breaks. It must be repaired quickly (days, not weeks) before you run it out of coolant, and overheat the engine.