Details: Like all Battle Facilities, The Battle Tree has a number of different shop salesmen that you can buy and give your Pokemon items that will give them competitive advantages at the Battle Tree. These Shops even have Mega Stones you can buy for all Pokemon in the Alola Pokedex that can Mega Evolve (this excludes a number of other Pokemon that can Mega Evolve since they aren't found within the game).
Left Shop Salesman Items:
King's Rock - 32 Battle Points
Deep Sea Tooth - 32 Battle Points
Deep Sea Scale - 32 Battle Points
Dragon Scale - 32 Battle Points
Up-Grade - 32 Battle Points
Dubious Disc - 32 Battle Points
Protector - 32 Battle Points
Electirizer - 32 Battle Points
Magmarizer - 32 Battle Points
Reaper Cloth - 32 Battle Points
Whipped Dream - 32 Battle Points
Sachet - 32 Battle Points
Middle Shop Salesman Items:
Toxic Orb - 16 Battle Points
Flame Orb - 16 Battle Points
Iron Ball - 16 Battle Points
Ring Target - 16 Battle Points
White Herb - 32 Battle Points
Mental Herb - 32 Battle Points
Power Herb - 32 Battle Points
Focus Sash - 32 Battle Points
Air Balloon - 32 Battle Points
Eject Button - 32 Battle Points
Weakness Policy - 32 Battle Points
Choice Band - 48 Battle Points
Choice Specs - 48 Battle Points
Choice Scarf - 48 Battle Points
Life Orb - 48 Battle Points
Rocky Helmet - 48 Battle Points
Assault Vest - 48 Battle Points
Safety Goggles - 48 Battle Points
Terrain Extender - 48 Battle Points
Protective Pads - 48 Battle Points
Right Shop Salesman Items:
Gergarite - 64 Battle Points
Scizorite - 64 Battle Points
Pinsirite - 64 Battle Points
Aerodactylite - 64 Battle Points
Lucarionite - 64 Battle Points
Kagaskhanite - 64 Battle Points
Gyaradosite - 64 Battle Points
Absolite - 64 Battle Points
Alakazite - 64 Battle Points
Garchompite - 64 Battle Points
Sablenite - 64 Battle Points
Metagrossite - 64 Battle Points
Sharpedonite - 64 Battle Points
Slowbronite - 64 Battle Points
Glalitite - 64 Battle Points
Salamencite - 64 Battle Points