Since you're an independent, get used to this...it will be ongoing.As a fellow "indie," my own practices for keeping enough materials on hand are these:
1. Keep yourself informed about what's available. Either call the exhibitor relations people at the film companies (who can often ship you materials directly, bypassing TES completely) and/or subscribe to a service such as I.M.E. (www.imeonline.com), which will keep you updated as items become available. The most important thing is to develop a relationship with your exhibitor relations people.
2. Order materials EARLY for everything that even looks remotely playable, even if you think you might not play it. I skipped ordering stuff for "Musketeer" because I thought it looked like a bomb...then when I found we were going to play it, there were no one-sheets available. You will spend more in shipping charges with some film companies, but you can combine several items in one shipment to save charges. Even if you have to pay more for shipping, that's better than getting stuck with no promotional stuff when an unexpected blockbuster hits.
3. Subscribe to the exhibitor relations websites. There, you can order online and not have to deal with anybody (my favorite approach), and, hot tip: when you order through the websites, you don't have to pay for shipping! Currently there are three such sites:
www.exhibitorrelations.com (Universal) www.sonypicturesreleasing.com (Sony) www.foxexhibitor.com (20th Century Fox)
I'm sure the others will follow suit eventually.
4. If you're really desperate for a trailer or a one-sheet, post a message here on film-tech. I've been helped out several times by other members, and I've helped out a couple myself.
5. Get to know other theatre managers in your area, especially the large chains. They sometimes get bombarded with stacks of trailers many of which they never use and would probably be glad to give you what they can.
Good luck and have a great time!
Being able to view the trailers on line is good, but only for those with fast internet connections (not available here in stix-ville).