NFLPA rule: Junior year after their last regular season college or conference championship game (excluding any post-season bowl game) or December 1, whichever is later. Known as Dec 1 rule.
In some states I wait until their final collegiate game (senior year) is over (may be bowl game) in order to not have to register as student athlete-agent (common UAAA rule) in that state since their collegiate career is over to make first contact.
This probably puts me at a disadvantage in some circumstances by coming to the party "late" but it has not prevented me in past from contacting any targeted clients.
Contact info can come from anywhere, social media, friends of friends, other players I represent.