Cetus Wisps are commonly needed for crafting important gear, ranging from Operator Amps to Zaw components, as detailed here. You can find Cetus Wisps throughout the Plains of Eidolon on the edges of water bodies at any time; however, they spawn in greater numbers during Night cycles. You can also purchase 1 for 2,000 standing from The Quills upon reaching their max syndicate rank. Here is a guided Map containing the recommended route to farm wisps.