Marc Liyanage's pages have what you need. There are .pkg installers and startup scripts as well as instructions for installation from source. There's also useful stuff about php and postgresql and, er, ruby.
The downloads on mysql.com that someone mentioned are _not_ what you want. They're for pre-10 versions of OS X - server 1.x and the public beta. The old OS X server was just a Rhapsody relic and should never have been called that: it has no real relation to the BSD-based server X which is now available.
nb. i've only done this on client os x: be sure to read the apple docs and make sure you're not trampling on some custom admin script or other.
i don't think you can have searched very thoroughly, by the way. try this one.
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