They aren't listed by name, rather, by random combinations of letters/numbers...you'll have to open each folder individually to find the app you're looking for.
AppStore apps are not downloaded and stored as .ipa's on the iPhone, they are just installed automatically. The .ipa's are kept on your computer by iTunes, but the location escapes me at the moment (plus they are different locations in Mac and Windows computers).
Don't know what else to suggest...I just tried it by deleting an app i had in iTunes, re-downloading it, and when it showed up on iTunes under my apps, I right-clicked the icon, chose the "show in Windows Explorer" option, and it showed me where the IPA was located on my computer.