expression, godan verb (-ru)
obtain; get hold of; come into possession of
Used when something desired becomes available or is acquired, often with a nuance of effort or luck. The subject is the thing obtained, not the person.
やっとチケットが手に入った。
I finally got hold of a ticket.
この本はなかなか手に入らない。
This book is hard to come by.
手に入れる is transitive (someone obtains something), while 手に入る is intransitive (something becomes obtainable).
Compound of 手 (hand) + に (particle) + 入る (enter). Literally 'to enter one's hand', expressing acquisition.