You can get both of them at the bazaar, but they're very rare and expensive there. But there's a few farming spots for both of them.
Assuming that you have access to Grizzleheim, theres a lake in a cave of Mirkholm keep. There's like 3 or 4 spawning spots for frostflowers.
Start off at the wraith realm, grab the reagents, and work your way up until you get the amount you need (or until you get bored lol).
For black pearls, you can start a pink dandelion garden. But since it takes a while for them to harvest, black lotuses can be found in all over Mooshu. But personally, my favorite farming spot is the Savannah in Zafaria.
Black pearls can be found all over Mooshu, or harvested from Pink Dandelion plants. (They are frequent drops in battles, especially Krokotopia and, oddly, Celestia.)
Frost Flowers can be found in Grizzleheim, and any snowy area elsewhere in the spiral. They can also be harvested from Fickle Pickle plants, available from the gardener mole in Golem Court, Wizard City.
Alia Misthaven