Temerian trousers are light armor trousers in The Witcher 3: Wild Hunt, part of the Temerian Armor set free DLC, and can only be purchased from Bram in White Orchard.
Whether due to a bug or intentionally, the upgraded New Game + version can be purchased in a regular game as well. At level 29, it will replace the level 4 version in Bram's inventory. Similarly, it cannot be purchased before reaching level 29 even in a New Game +.