Mallowsweet herbs are needed to activate Merlin Trials, you can grow it's seeds or buy some from vendors.

Vendors

The Magic Neep in Hogsmeade sells 2 Mallowsweets for 100 gold each, but you can buy the Mallowsweet Seed and grow more for 200 gold.

Grow Mallowsweet

Mallowsweets can be grown in Potting Tables in the Room of Requirement.

• Each Mallowsweet Seed gives 5 Mallowsweets in 10 minutes.
• Potting Tables in the Room of Requirement will be available while progressing the main storyline. It's a little bit later than flying brooms quest.
• You only need 1 Mallowsweet Seed, which means every 10 minutes and without buying a new Mallowsweet Seed, you will get 5 Mallowsweets.