Freeze PLugs in the Head

I have recently paid a local mechanic to replace all the freeze plugs in my Nova due to a small leak. After all the freeze plugs had been replaced I tried to drive the car, made it 5 minutes down the road and the car started smoking and it smelled weird. I took it back to the shop and they told me that the car had a couple of freeze plugs in the head of the car and they could not find the parts to fix it so when the other freeze plugs got changed it put pressure on the ones in the head and made them leak. The problem Im having is the shop told me that the parts I need are no longer made and they cant get them, So does anyone know what size the plugs are and where I might be able to find them. The engine is a 250 straight six

thank you