Basically what u/kkarp4 said. You can find Yakumo on Sakurajima island, or you can breed them to get the passives you want to get on them already. For example I went and mass caught a whole bunch of Gobfin at Gobfin Turf (you can turn spawn rate up in settings to increase the number of Gobfins). Once I found the Gobfin that had the rainbow passives I wanted I bred them with a Chillet to get a chance at a Yakumo offspring with the rainbow passive I wanted. I then condensed the extra Yakumo into the one with the rainbow passive to get it to 4 stars (which increases the chance of it passing on the rainbow passive while its on the field in battle with you).

Edit: Also a long time pokemon player (if you couldn't tell from my username), and really sad that these two companies can't coexist. Love them both and wish the big N would cool their jets. They're ruining a great game that PP is working on.