You can only download iPhoto 9.6.1 or Aperture 3.6, if they are in your Purchase History at the App Store. Both applications cannot be downloaded outside the App Store.
See Barney's User Tip: Can't Update iPhoto because it is not available in the App Store
But you may want to check the filename extension of your iPhoto Library or Aperture Library. When Photos is opening a library it changes the filename extension to mark that it has been already migrated.
- if the filename extension is .migratedaplibrary change it back to .aplibrary
- if the filename extension is .migratedphotolibrary change it back to .photolibrary
Then try again to open the library in Photos. Launch Photos while holding down the options key ⌥ and wait for the library chooser dialog to appear. Then release the options key and select one of your older libraries to browse it.