Potassium Bromide is an inorganic chemical. Once bought it does not deteriorate. Some of my stash is over 40 eyars old , and still works fine.

It comes as white granules, that over time, under exposure to atmospheric moisture, tends to form rock hard lumps.

For precise measurement, and resonable mixing times, I keep a small stone mortar and pestle to break up the lumps.

I have seen it sold by chemical supply houses (but at a higher purity grade than we really need for photo use).

Currently I buy it mail order from Photographers Formulary. There must be other sources, but the Formulary have such a great selection of the stuff, there is always the ability to add other things to the order there.

I would suggest you consider buying and thoroughly reading The Darkroom Cookbook by Anschell and Troop. It will give you good insight into the different ingredients that go into a developer, as well as other photo related baths like fixers, tonere, etc.

Jacobsen's 'Developing' is also good, but the formulas, packed into tables, are not as easy to read.