Look closely at some of the better multi-tools, and they often have valve core removers -- sometimes a spoke wrench does double-duty. So if you are carrying such a multi-tool, you've got it covered.

Otherwise you get a tiny plastic valve core tool with multi-packs of valve cores, which you will end up purchasing if you run tubeless. Those little plastic tools weigh nothing and can be added to a patch kit, taped to a tire lever, whatever.