Most of the hikers that I know buy when stuff's on sale, and dehyrate their own. (but I think they do more fruits than vegetables, for the energy needed for hiking)
As for where you buy 'large' amounts, it really depends on what sort of quantities we're talking about. I'd really suggest trying lemontwist's advice, and looking for a local food co-op first.
If you don't have one nearby, you might try some of the 'disaster preparedness supply' websites. Most sell full meals, but if you dig through the websites (or mail order catalogs, in case you're concerned with the government knowing that you're making these preparations), you'll find that some have bulk dried foods in either resealable cans or bagged, and each site will stock different varieties and sizes (anywhere from 1.5 oz to 2+ lbs) ... and remember, that this has no water in it, so 2lbs could be 20-40 cups); many mention '#10' can, which is roughly 13.7 cups (~3.2L)
Another option is to look for various dried 'soup mix' (also available from the emergency preparedness folks, and in some grocery stores), but check to see how much salt's been added. A health food store's going to be more likely to have a low-salt option than your general grocery store.