Hi Aliciaj8, I'm guessing you're just starting out. I agree with you; gather lots of mermaid scales can be difficult. They can be found in a few places, but constant collections from ponds is your best source unless you have any of the other items or the gem only Mermaid Pond which is a guaranteed source. Try and have around 5 or more ponds if you can afford the space to increase the chance of m/scales dropping. Good luck and welcome to the forum!
here's an extract from the Inventory Guide on m/scales:
Found: mermaid pond, ponds, wells, swampy pond, trade with Belladonna, Castle Garden, Princess Lorelei's Fountain, Faerie Fountain, adventures, Glen the Frog Prince, Swamp Kitty, Aquamarine Dragon (Baron's Tent), Aquamarine Alicorn (Baron's Tent), Flower Fountain (Baron's Game Tent), Celestine castle, Sirene, Azura, Penguin, Blossom bridge, Koi pond, swan pond, Princess Lorelei, Spring Fountain, Wisp Garden, Aquanther Cub, Mermaid Fountain, Lion fountain