Since the cap is plastic, I'd guess that's worn rather than the steel res. Chances are you can get one from the the internet. Yep, $4...
You can get a kit to reseal the pump as mentioned above. If you want rebuilt pump, go to the nearest parts store with the usual info.
The pumps are typically the same over many years but the res varies as does the shaft and pulley mount. The pulley either has a nut or simply presses on.
https://www.amazon.com/Dorman-82581-...XNWVB4X5ZWEEWQ