General:
* Consistent
* Regular
* Standardized
* Homogeneous
* Identical
* Similar
* Unvarying
Referring to clothing:
* Outfit
* Apparel
* Attire
* Garb
* Costume
* Dress
Referring to something unchanging:
* Constant
* Steady
* Invariable
* Unchanging
* Stable
More specific, depending on the context:
* For a school: School clothes
* For a military: Military attire
* For a team: Team jersey
* For a company: Workwear
The best synonym will depend on the specific situation and the meaning you want to convey.