The good news when you are starting out in the bouncy castle business - you don't need a ton of space. Most people start out in their 1-car sized garage at home. Makes things super convenient to access the equipment on a dimes-notice and also it will save you a lot of money.
Once you reach a level where you need more space, a good option could be a local storage unit which can be rather an inexpensive monthly fee. Just ensure you have 24 hour access to your unit.
Also, it is a recommended you store your castles on skids. This will help circulate air underneath and keep it off the ground for any potential spills or floods.
Wherever you decide to store your inflatables, keep in mind - you will beed a place to inflate your castle(s) for cleaning, drying and/or any inspections. Again, most start out doing this in their front or backyards but this is something to think about when you are deciding where you want to store your castles.