While I'm normally a fan of compiling from source, Slackware provides packages for both ncurses and MySQL. I'd use pkgtool to install them from your original installation media.
That said, if you're interested in compiling from source, I'd do the same for ncurses. It's freely available from the GNU mirror sites (this
URL came up first). I've never had much luck with RPMs and Slackware, which really isn't surprising. I've always had good experiences on Slackware compiling from source.