https://vxheaven.org/ - "This site contains a massive, continuously updated collection of magazines, virus samples, virus sources, polymorphic engines, virus generators, virus writing tutorials, articles, books, news archives etc. Even the viruses for the platforms you've never heard of. We also offer free hosting for virus authors and groups.".
They've got most of the ancient MSDOS viruses [virii?], and most of the Win 9x ones as well. Pretty much everything there is so outdated that every AV should have signatures in their db against them, so I don't see any problem with posting this link today. Also the website is setup in a way that it's very difficult to accidentally infect yourself, as any binaries are zipped up and in a very obvious section to avoid confusion.