It's very hard to find short ends of 100D. In fact it's getting harder to buy 100D at all - the only remaining 35mm size is the 400ft roll on a core. I'm going to go out on a limb and guess that Kodak canned up a master roll of it for some cinematographers that were demanding it, and once those all get sold, there will be no more. It's still pretty popular with the 16mm and 8/super-8 crowd but I'm betting it's virtually unused by 35mm cinematographers these days.
At $475 for 400 feet I considered it no bargain at all compared to buying normal still camera film from Kodak... but now that it's the only reversal film left maybe that price doesn't seem so bad. But grab it while you can, like I said I bet once the frozen supply at Kodak is gone, it's gone. (Movie film is not given an expiration date - it's assumed to be a short path from Kodak to the camera to the lab - so they can and will just keep selling it for however long it is until it's gone, if my theory is correct.)
If you send it off to a lab for processing, you should make sure they are OK with this (from the data sheet ):
DuncanNote: KODAK EKTACHROME 100D Color Reversal Film 5285 / 7285 contains special sensitizing and filter dyes that improve color reproduction. Because these dyes are designed to rinse out of the film during processing, they will change the color of the first developer, the reversal bath, the final wash, and the final rinse. This solution discoloration is only cosmetic. It will not affect sensitometry or the quality of any Process E-6 film or control material. However, the solutions will cause splicing tape and processing equipment (rollers, racks, etc.) to have a pinkish color. The pink dye residue can easily be washed off processing equipment by following normal maintenance procedures.