So the plastic bypass fitting broke off into the lower intake... something like this?
The flexible, twisted bypass hose that runs from the water pump to the lower intake is $3 at Autozone. As you found out, the threaded fitting is a dealer item. The replacement part will be metal not plastic.
You can try using a wet/dry vac to suck the chunks of plastic out of the lower intake. I kind of gave my lower intake an enema using water and a turkey baster, then I sucked the bits out with a shop-vac. Then I drained my coolant and refilled. Then I took the car to the shop to have the coolant properly flushed.
On my 93 SE I replaced the O-ring where the metal heater pipe goes into the lower intake. It was a standard O-ring available at Home Depot... I want to say it was a #53 but I'm not sure.