Tyler,
My suggestion is to begin saving now while you research your area. You should know where all the facilities in are located in the market you are looking to purchase, along with what average rents are, and what the average vacancy rate is for the facilities. GoogleMaps and Streetview are great tools to quickly locate a large number of facilities at home, but once you find them drive to the ones that interest you to check them out and talk to the office manager if the property has one. While your out driving, take down the contact numbers for the facilities because I know in my area storage facilities listed for sale are few and far between so the best way to get your hands on one is to call the owners/managers directly and ask them if they are interested in selling.
A tool I also use in my property search is my county GIS system. It shows the parcel boundries, owner, site size, and other items, depending on the county, for all properties in the county. I would Google your county's name with GIS at the end to see if your county has one available. I use it to see if an existing facility has an excess land with it, and if the area I am searching in is under supplied, I use the GIS system to find the owners of vacant land parcels in good locations for a facility.
As far as how much you need to save, that all depends on the size of the facility you are buying. I'm sure since you are on BiggerPockets you know that the value of the property is a function of it's cash flow, but a traditional lender will typically lend at 75% of the appraised value which is typically based on the income approach (ie the cash flow) so you'll likely need to save 25% of the value. The same is true if you plan to build a new facility, but you must have a good grasp on your construction numbers and the income approach valuation. Just because a facility costs $500k to building (including land cost) does not mean the property is worth $500k.
Best of luck in your search