You can find them on Amazon.

Young Justice (2011-2013): Book One: The Early Missions https://www.amazon.com/dp/B07YCWRSFR/ref=cm_sw_r_cp_api_1F9ZTNV0NB8Q2S8SF7VC

Young Justice Book Two: Growing Up https://www.amazon.com/dp/1779509243/ref=cm_sw_r_cp_api_i_WJWRAQ41GMMZGEK20SM6

Or ask in your favorite/local comic book store. They also available for digital purchase or subscribe to DC Universe if available in your country, you can also find them on EBay.

And there’s going to be another volume (6 issues) that will start dropping a week after Phantoms is done airing and it’s called YOUNG JUSTICE TARGETS