For a delta update of one version of a resource to the next, the resource itself is the dictionary (i.e. JS file).

For stand-alone dictionaries, the brotli code on github has a dictionary_generator that you can use to generate a dictionary. You give it a dictionary size and a bunch of input files and it will generate one. I have a version of it hosted on https://use-as-dictionary.com/ that you can pass up to 100 URLs to and it will generate a dictionary for you (using the brotli tool).