Many companies (Public Storage, Extra Space, CubeSmart, local) — various unit sizes, climate control, 24/7 access (some). Monthly rental.
Services like LuggageHero, Bounce, Vertoe — hourly/daily rates at shops, hotels, transit hubs. Good for bags, not furniture.
Some apartment buildings lease unused closets/storage cages to tenants. Ask landlord/management.
Free, flexible — if you have trusted contacts with space.
Neighbor, StoreAtMyHouse — rent space in someone's home/garage. Often cheaper than commercial.