I don't think they make a 1000 calorie bar, I think they are typically packed as individual 400 calorie bars. They make a stack of bars and wrap them together as 2400 or 3600 calories bricks. It seems like that's the standard with many of the survival rations I've seen. Here's a pointer to a site that has them:
http://www.emprep.com/food%20water.html Also on that site are "New Millenium Bars", which come in some MRE's. They taste ok, are a little dry, but, also packed with calories. For some reason, even though they are reasonably healthy, and vitamin filled, our boy scouts will eat them on hikes. I guess if you're hungry enough...