Actually you can download the latest snapshot builds from nexus.pentaho.org (You can also download the latest nightlies but you'd have to guess the version numbers, so that's not very practical - working on a better way).
All the CE assemblies are published as artifacts on nexus so they can be requested as dependencies in maven or ant.
something like this (in ivy) for the pdi client should get you the latest snapshot:
<dependency org="org.pentaho.di" name="pdi-ce"
rev="9.0-SNAPSHOT" transitive="false" changing="true" conf='default->default'>
<artifact name="pdi-ce" type="zip" />
</dependency>
and for biserver
<dependency org="pentaho" name="pentaho-server-ce"
rev="9.0-SNAPSHOT" transitive="false" changing="true" conf='default->default'>
<artifact name="pentaho-server-ce" type="zip" />
</dependency>
similar stuff for maven.