You can do this one of two ways: the way the game intended, which is to find all the letters and do all the side missions (click here
for a guide to the letters and here
for a guide to the side missions) or the easy way, which is to simply input all the phone numbers that unlock the collectables. The list of phone numbers can be found here
, but only the ones that say "collectable" next to them are the ones you need for this. There are only 73 phone numbers, because the remaining 27 are obtained from finding letters in the underground. Use the first link above for finding these letters, then once you are back in the normal world, mail them to yourself from a mailbox.