You can buy a card from any of the places listed here - https://cc.a-kent.com/Everyone/SSS/SSSGoster.aspx?key=e3084102-5d9a-48ff-a0e8-6ff93a621741

You can purchase a 3-ride card for 5TL from any of the booths located near major Tram stops but be aware that the booths are only open from 07:30 - 19:00 (10:30 - 19:00 on Sundays).

The costs for journeys can be seen here - https://cc.a-kent.com/Everyone/HatUcretleri.aspx

You can open an online account to get bus GPS location info and top up the card online here - https://cc.a-kent.com/Everyone/KayitliBasvuruKullaniciTanimla2.aspx - you can also topup the card at the offices listed in the first link.