Instrument- Rent or buy it from a local music store.
DON'T BUY INSTRUMENTS FROM AMAZON, EBAY, WALMART, COSTCO, ETC - THEY ARE TERRIBLE!
Bach 6 1/2 AL (small shank) mouthpiece or Yamaha 48 (small shank) mouthpiece - Highly recommended! A lot of beginner baritones come with a mouthpiece that is too small and it can make learning the baritone more difficult as a result. All local music stores should be able to get you one of these mouthpieces. If not, order it online.
Music stand - to use at home for daily practice. Don't get the cheap, flimsy folding metal music stands - they are garbage. Here are a few suggestions - music stand 1, music stand 2, music stand 3. Your local music store will also have similar options.
Instrument maintenance / care kit - Local music stores will likely have something similar.
Extra valve oil - I recommend Blue Juice or Al Cass brands.