How to get recruitment ticket?

There are various ways to buy recruitment tickets but you can earn them in the game for free. Every day you will get some you can use for free. 1 Super (orange) Ticket, 1 Advanced (purple) ticket and 3 Normal (blue Tickets).

Completing the daily quests up to 260 points gives you 3 normal tickets for free, and completing it up to the full 500 points will give you 1 advanced ticket. Clash of zones rewards include 1 (or 2) Super Tickets for gaining the 3rd box each day and also the reward for winning a CoZ event each day will include a number of super or advanced tickets depending on your level and the league you are in.

There are also other occasional events to look out for that have tickets as rewards.