The uniform shop that USED to be located on the 4th deck, 2nd corridor (4C251) has moved. The new location is 5E1085 (basically, it's directly above the Metro entrance on the 5th deck).
Update July 2013: I'm back at the Pentagon now, and they've finished all the construction / remodeling, so it's pretty easy to get around. You can come at the uniform shop from just about any direction now.
The good news, the new shop seems bigger to me. It's hard for them to offer a complete selection of uniforms for the USN, USMC, USA, and USAF all in one place, so they don't tend to have a LOT of inventory of any given item. Still, I was able to walk in and find a new khaki shirt in my size pretty quick.