What are the ways one can get 50% clash of zone tickets?

The tickets come in rewards. There are 3 alliance reward boxes for each Clash of zones event day. Each has 1 50% ticket. Then you will also find then in reward box 6 for your individual rewards each day and a 100 % ticket in box 9.

If you haven't unlocked all the reward boxes for CoZ then concentrate on researching in Zone Commemoration. If you get to the end of this research tree then you will get double rewards as well.