If your willing to pay to play, then storageunitauctionlist.com is your best resource. Otherwise stick to your regions newspapers, and scan them every day.
Right off the bat, you know my opinion is biased; however, one could reasonably assume that there is a problem when you visit StorageUnitAuctionList's website and the first question in their FAQ's is " Q: How often do you update your data? I am finding auctions not on your list!"
I respectfully disagree with the statement that they are your best resource. It depends on the state you live in, whether you have to subscribe to the newspapers, how many storage facilities are in your area, etc. In Texas, they're not even in the same league as AuctionsTX.com. In California and several other states, you can find a majority of the information on the auctioneer's websites for free.
In some areas where the auctioneers and newspapers don't publish their auctions online, they may be the best bet because they do call the facilities; however, if you live in a small to medium size town, you're better off making the phone calls yourself, you will get the most accurate results this way.
We were actually contacted by StorageUnitAuctionList's former call center manager. Apparently, they outsource their phone calls to India. Can you image how irritating that would be if you were a storage facility manager and you received phone calls from people you could barely understand every few weeks? Would you give people who annoy you accurate information?