A way that I used since the begining was searching in the spirit tree location, since there is a 50/50 chance to get spirit orbs. And you have to mate the spirit in order to customize and be able to mate with them in the mating mode. Also giving them clothing is another way of customization.
Viewing post in The Spirits of Sabil (+18) comments
Okay you could also fish and sell them to Sean as well I think it's 10 orbs per fish. How do I unlock the area where Haru is at? I guess I just progress the game enough but I haven't played the game in awhile back when I did Haru wasn't in the game nor the cave area. I also find it hard to interact with the NPC spirits as you basically have to guess what to say and go from there as you don't know which answers they like I only did it right once so far but I've tried two times in Hazel grove after quitting the game the first time I found one and in both scenarios I made them dislike me so I'm not sure what to do with that event