The new staging intermediates are available via their AIA URLs:
http://stg-e5.i.lencr.org
http://stg-e6.i.lencr.org
http://stg-e7.i.lencr.org
http://stg-e8.i.lencr.org
http://stg-e9.i.lencr.org
http://stg-r10.i.lencr.org
http://stg-r11.i.lencr.org
http://stg-r12.i.lencr.org
http://stg-r13.i.lencr.org
http://stg-r14.i.lencr.org
The production URLs are e5.i.lencr.org and similar (without the stg- prefix)
Generally, intermediates are provided via ACME when a certificate is issued, but there may be some circumstances when other intermediates might be needed.
We are working on updating our documentation right now.