It depends on the sound the vowel makes. There is a "yuh" sound at the beginning, if it were pronounced "oooniform" then it would be "an uniform"
Ceci est mon uniforme scolaire
I use to attend a school where we had a dress code. I had nothing against it, but I didn't like it very muc. There were barely any clothes my size everythying they had was uncomfortable. It's bad...
Well, depenging on your uniform is depending on where you can get yours. Most stores will have them.
The ACU uniform adopted in 2005 is. The current Multi Cam uniforms being issued to units deploying to Afghanistan are not.