Trinkets can be dropped by monsters, obtained in Skull Cavern Treasure Rooms, or purchased from Alex in the Desert Festival.

Sadly, can only be found after Combat Mastery is claimed. Combat skill is increased by fighting monsters, and note that killing monsters on the Farm grants only 1/3 of the standard amount of experience.

Consuming certain foods or drinks, wearing particular Footwear or Rings, and choice of Weapon can all give Stat Bonuses that affect the player's Combat ability, and therefore allow the player to progress much faster to mastery.

When the player accumulates enough mastery points to unlock a new mastery, the game shows a message saying "You've reached a new level of understanding...". The cave's pillars are then available for unlocking a mastery. After visiting The Mastery Cave, you must have unlocked a new equipment slot for trinkets.

The player can obtain the same trinket multiple times, but they don't stack.
All Trinkets, with the exceptions of the Basilisk Paw and the Magic Hair Gel, can be reforged using an Anvil. This randomizes their stats and costs 3 Iridium Bars. All can also be sold for 1,000g.

To get the Magic Hair Gel, you need to assist to the Desert Festival, that happens every year in the Desert on Spring 15th, 16th, and 17th. Several challenges and attractions offer the ability to earn this festival's currency, Calico Eggs.

The Calico Egg Merchant sells several accessories and furniture items, as well as various usable items. Some items are sold every day, while others are only available on certain days. There, Alex would sell you Magic Hair Gel, for 100 Calico Eggs.

The trash bin next to the Calico Egg Merchant's shop can be checked and contains Calico Eggs once per day. 😉