@Frost Knight In Woodelves and Sorcerers and Dragons, your best mana source is Festival Merchants. You craft these in the magic academy. Keep an eye out for them. I literally crafted Festival Merchants every single time they appeared while I was doing these chapters. I replaced every single Weeping Willow I built with FMs. I think I topped out at something like 12 or 13 Festival Merchants. If you ARE still using Weeping Willows, the Arcane Library you unlock in Sorcerers and Dragons is an upgrade. But it isn't as good as a Festival Merchant.
Another thing to keep an eye on is what evolving buildings and set buildings you can upgrade to produce mana. For example, in the Moonstone Library set, the Mana Plant, Gum Tree, and Moonstone Gate are very efficient mana producers for their size. Since they are so cheap to upgrade, they should be a priority. The Forbidden Ruins and Pilgrim's Manor sets also have some good mana producing buildings if you have any of these sets. Some of the evolving buildings which make mana at your level include Wise Golem, Stonehenge, Polar Bear, Red Panda Master, Queen Fairy's Retreat, Watchful Winter Owl, Flower Goblin Ephinay, and Boblin's Express Service. This list is off the top of my head, so I am sure I forgot some. Also note that some of the buildings I listed will stop producing mana and will start producing Divine Seeds when you upgrade them to Halflings level (that is the chapter after Sorcerers and Dragons).
In events, some of the best mana producing buildings you can win are Magical Mana Hut, Manta Ray, Burning Pool, Lava Egg, Lunar Factory, and Pond of Autumn. Look for them as daily prizes in future events. In the ongoing Season of Joy, there are some buildings as reward that make mana at your level. Sharkapalooze, Pond of Spring, Ice Temple, Barrel Crane, Float of Bubbles, Temple of Embers, Aurate Willow, Eldarisil's Sawmill, and Hidden Lagoon are all buildings which are being offered as prizes in the Season of Joy which will make mana at S&D level.