Just finished a shulker farm so the storage doesn't become a problem in the near future.
The farm is located at around x: 1960 z: 440 (it used to be a city...). If you go through the end gateway with the sandstone stairs you come out in a gateway that is in a straight line to the west of the farm, just keep going east and you'll find it. It is easier with an elytra, but in the future I'll be building a rail line to get there easily.
I'm not sure about the rates, but it should produce a good amount of shells. You are welcomed to take any shells you need and/or afk here to produce some more.
Well, to get the shells you just need to stand around the farm to keep it loaded and the shells come out on their own. Specifically, there are shulkers inside the farm that are constantly aggro to a snow golem, which makes them shoot their bullets, which hit them and damage them, causing the shulkers to split and generate more shulkers. When a shulker is damaged enough it dies and the shell gets collected by a water stream under the farm.
Awesome! Thanks for that explanation.