I've found really good bargains at Mervyns for things without logos. Unfortunately, everything except the shorts/pants for my son's stuff has to have the school logo on it, restricting us to the single company that the school works with. Now that gets expensive!
Good quality (and better fabric) but slightly higher prices from Land's End. I love their clothes but sometimes have to struggle with the pricing a bit.
The nice thing about uniforms is that once you get started I'm pretty convinced they are cheaper than buying lots of options. If you don't mind doing laundry a lot you can buy just a few pieces. If you are really tight on the budget right now, that might be the answer for the short term. No one can tell that she only has 1-2 pair of pants because they all look the same.
And if uniforms are new for your school, definitely have someone set up a uniform exchange system for next year. Our school is really basic -- drop off clean stuff you have outgrown and pick up anything you can use in the future -- no money involved. If you have a child with an easy size it can be really great.