There's a Trader Joes in Palmdale right off the 14 FWY, which will be your route up to 395 from LA. There's also a Target right across the street that sells all the cool drink mixes, a grocery store within a few blocks, and a Sports Chalet within a block where you can get freeze dried stuff there as well as Gu and Shot Blocks and hydration drinks. All of these places are normal prices and right next to each other with plenty of parking, something you might not get in LA.
Joes has a great selection of nuts, trail mixes, oatmeal, food bars, packaged meals, etc.
After that, you are into the big void of the Eastern Sierra. There are stores in Lone Pine, but you will pay more and not have selection.