You can buy a fragment that gives essence using clusters. There is one specifically for DR called Pure Essence. Get the right one. You should have at least a small amount of clusters from ranking up. You can't use this type of essence if you are only rank 10 though.

Farming them isn't hard. You can do a castrum run for deep essences, kill a few mobs near the aethernets for normal essences or buy them from the market board.

The content says before you do it to review your Lost Action setup, because the content is tuned with the assumption that Lost Actions are being used, including essences. Ignoring this makes the run slower and less fun. My first run felt like it took over an hour and I didn't do it again for months I was so put off.