Comment by noelwelsh

Here is one company that provides the gowns for many UK universities: https://www2.edeandravenscroft.com/non-ceremony/. I could be kitted with the appropriate gowns for my UK degrees for a few hundred squid each.

Academics, who attend many graduation events, often end up buying their own gown. I believe you are supposed to use the gown of the university you graduated from, so they must be for sale somewhere.

Once you have your own gown you can customize it. I haven't seen any profs with LEDs additions to their gowns, but adding extra pockets to hide a book or snacks is fair game.