If they were popular enough (like all the examples you’ve named), they probably have been converted to a more modern format already (like all the examples you’ve named).
You could also download the files and still watch them on a local machine using flash player to enjoy them in the original format, assuming the .FLVs themselves are still available. Makes me wonder if Newgrounds still has any of that somewhere, even if not accessible by the public. They’re still around, but they’re modern videos now.