What you are looking for is the bypass fitting - it is GM part #88891743. Metal is preferable to plastic because the plastic ones will eventually degrade and leak, and when you try to remove it, it will break off inside the lower intake, causing you no end of grief:
has them for $7 plus shipping. Most dealers should be able to get you the metal replacement. It will be $15 to $25 at the dealer.
You could replace the bypass hose itself while you're at it - it'* only about $3 or so at Autozone. It'* the hose that runs from the water pump to the lower intake.