You can transmute pearls to golden pearls, or you can find them when harvesting pearls.
If you have access to the Lunarium that would be the place to look. The portico area is easy access to the entrance and there is no fights. Within that area, you will find 2 pearls, 2 kelp, 1 shell, and a wooden chest that sometimes gives a reagent. One run through there takes me less than five minutes. Go around the portico area then reset the instance by going back out the entrance. Then simply re-enter and repeat the process.