It's either replace the pump, highly unlikely you will be able to find a replacement check valve, though you could take it apart and try to clean out any debris in it keeping the valve open which allows water to flow back to the tank. I had the same situation some years ago and installed a check valve in the PEX on the pressure side of the pump which resolved the problem. It's still installed, even though I've since replaced the pump due to a different issue. In the photo, it's the brass fitting in the lower center of the photo.
Bob & Becky
2012 Chevy 2500HD D/A CC 4WD