There are several parts to a water pump, and only the housing, pulley or fan hub, and sometimes the impeller are re-used during a rebuild. The shaft, bearings, and seal come as a package unit which presses into the housing. Then the impeller and fan hub are pressed into place. The seal is a very oddball design, a spring-loaded "mechanical seal" consisting of a ceramic disc with a carbon ring spinning against it, held together by a stiff spring. The seal is very fragile, and can be damaged during assembly. The shaft/bearing/seal cartridge is the part that's very scarce these days. I'm pretty sure high volume rebuilders probably have a source, but they're pretty secretive about where they're coming from!
Jerry
