Business casual usual means khakis, slacks, or knee to calf-length skirts (no jeans, no shorts, no cropped pants, and no mini-skirts). Button-down shirts (of any sleeve-length) are always appropriate. You may also be able able to wear higher quality knit shirts or sweaters, like a good twin-set (but nothing that looks too much like a t-shirt and nothing that is too baggy or too tight). Once you've worked there for a month or so, you'll probably have a better idea of what is acceptable. Also, when in doubt, dress up. It's generally better to be over-dressed than under-dressed. There is a saying--dress for the job you want, not the job you have.
Landsend and
L.L. Bean are both great sources of business casual wear. Check out the Landsend Overstocks for some especially good deals. So long as you stay away from the jeans and casual t-shirts, it would be hard to go wrong. You can also get Landsend at Sears.