It's in the Goldenrod Tunnel's warehouse, next to the Radio Director. This is the only Sleep Talk TM in the game.
TM082 (Bulbapedia)
http://answers.yahoo.com/question/index?qid=20100410212819AA54q7u
http://mycheats.1up.com/view/section/3172942/28732/pokemon_heart_gold_soul_silver/nintendo_ds
http://www.psypokes.com/hgss/tmhm.php
These kind of things are easy to look up, here are locations, but it's not hard to google.
