...I just returned a batch of Continental tubes (which are pretty high quality products), because for some reason this batch all had defective Presta valves.
I had installed four of them already, so those I kept, but just replaced the valve mechanism. You can buy those separately, for both Schrader and Presta valves.

The usually come in quantities of a dozen or twenty. Or you can save them from other tubes that get spoiled for some other reason.

The Schrader valve core replacements they sell at auto parts stores are usually pretty good quality, because people use them in car tires.