Depends mainly on where you are located.
Wherever you are, 486 is getting rare, but can certainly still be found, both targeted (people knowing what they have and priced accordingly) and untargeted ("come pick up this old trash for free/small amount"). Without knowing where you are, amibay.com is globally probably the best place to look. There are some 486 boards currently for sale there. Locally there may be other options.
And there's always "the community". I knew someone was looking for a very specific case. The case itself is nothing special, but he wanted to recreate the exact system he had in the late 1990s. I happened to see the case in a big junk lot nearby. So picked up the lot, confirmed it was exactly the case required, then got in touch. At the time I had no idea what if anything he had in exchange, but he immediately offered me a 486 board he wasn't using in exchange. But obviously, you need to be aware of people nearby looking for stuff, you need to keep your eyes open and you need a good dose of luck for this kind of thinng.
But failing the latter (which takes more time than you probably have at the moment), go to Amibay and look there, and tell us where you are located for additional options from locals.
