97 K2500 rear heater line question

Yesterday I had a sudden loss of coolant on the road. After briefly sending the temp guage to the peg, I limped to water and then home. (No damage)

A "T" fitting in the heater supply line had broken. I got a generic replacement at O'Reilly's. When I removed the old one it was badly deteriorated, crumbling like hard candle wax when squeezed. The return line has a similar fitting, so I bought a replacement for it in case it is made of the same crap as the one that broke.

My concern is: Are there other similar parts anywhere in my cooling system? If so, I want to change them. I'm especially worried about the valve (see "What is this part" post below) that controls the water feed to the rear heater.

While I've got you, what are the good online places to pick up parts for this vehicle? The one I always see recommended here gives an error when I agree to the terms of using their catalog.

Thanks!