How to expand your inventory
Every character gets 3 rows for free.
Fourth row can be bought for 1000 kinah from a Shugo in Poeta (Baevrunerk) or Ishalgen (Bacorerk). It's also possible to get this row in Sanctum, Pandaemonium or Abyss.
Fifth row can be bought for 10000 kinah in Sanctum, Pandaemonium and Abyss.
Sixth row can be bought for 50000 kinah in Sanctum, Pandaemonium and Abyss.
Seventh row can be bought for 150000 in Sanctum, Pandaemonium and Abyss.
Eigth row can be bought for 300000 kinah in Abyss.
Ninth row can be received from a 29 lvl quest in Sanctum and Pandaemonium.
This quest is super easy. You just need to talk to two NPC and cube is expanded for free.
If you want to get a tenth row you have to complete the Vindachinerk chain quest.
Thirteenth row can be bought from Black Cloud Marketplace.
At the moment fourteenth and fifteenth rows cannot be obtained.