If you really want to use snapshots, here's the repository they are deployed to. But I highly recommend using only stable versions, from Maven Central... :-)
<pluginRepositories>
<pluginRepository>
<id>sonatype-snapshots</id>
<name>Sonatype Snapshots</name>
<url>https://s01.oss.sonatype.org/content/repositories/snapshots/</url>
</pluginRepository>
</pluginRepositories>
Older Snapshots (prior version 5.X) are available via <url>https://oss.sonatype.org/content/repositories/snapshots/</url>
.