Several "exclusive" and community-driven archives house thousands of these legacy files: iPhoneOS Obscura Project : One of the most comprehensive libraries, containing over 17,000+ IPAs
Have a rare IPA to share? The preservation community is waiting. Do not let the bits fade to zero.
The market for iOS 6 IPAs is strictly one of digital preservation. As hardware capable of running these apps (iPhone 3GS, 4, 4S, 5) ages and batteries fail, and as Apple tightens the ecosystem against sideloading, the window to experience these applications authentically is closing. "Exclusive" IPAs are those that have been preserved by enthusiasts against the tide of forced software updates and cloud-delivery services.