I want to know if there is a way to take 10 while in combat for a specific skill (In this case Iaijutsu Focus).
I heard that there is a feat that allows you to do that but I couldn't find it.
The feat Hardened Criminal, from City of Stormreach (page 95), grants the creature immunity to attempts to Intimidate it and the ability to take 10 under any circumstances on a single skill, chosen when the feat is chosen.
Skill Mastery, a Rogue Special Ability in the Player's Handbook, lets you do this with 3+Int skills. It's not worth taking, considering the other options, for most rogue builds, but you could take it and it would do this. Being in the PHB this method will nearly always be theoretically available to you, whereas other methods of accomplishing this may not be.