The United Network of Newton Archives (or UNNA) is probably the perfect place to grab package software for your Newton.
But there are alternatives. Check out the Newton Package Downloader, a list maintained since 2001, for a not-so-organized list of packages available for your MessagePad. It’s arranged grid-like, and offers tons of software. But if you want something specific, you’ll have to do some searching. There’s no rhyme or reason to the site’s arrangement.
The other links “Ed” (as the site’s author calls himself) provides time out, meaning they may not be in operation anymore.
If the packages are downloading weird, you can check out this handy method for configuring your pre-OS X Mac to download the files correctly.