Yeah, they haven't updated the staging environment documentation yet. (I made a Github ticket for it a few days ago, but they've probably been too busy getting their new prod intermediates online.)
You can find one way to get the staging certs from this post:
So "Counterfeit Cashew R10" should be available from http://stg-r10.i.lencr.org/
But when you say "are failing", usually systems shouldn't care about the intermediates, since the server gets them from the ACME server when getting a cert and then sends them as part of the chain to the clients. If you're more specific about what you're doing, people here may be able to give you a better way. The intermediate you get can change with each certificate one gets.