We've been using this Safety First model for our 10 month old:
http://www.safety1st.com/product/detail.asp?ID=2754
It's sometimes difficult to push down into the tub, but my son seems fine in it. He can sit upright and play with the running water and toys. The arm clamp fits over the side of the tub. You can also adjust how it's facing.
I usually sit on a step stool by the tub to bathe him. Although not the most comfortable position, it's much easier on my back than some of the other tubs we've used in the past with our daughter.
I've seen some at Babies R Us, Target, just about any store that sells infant products. And of course you can purchase it online from the link above.
It needs to drain thoroughly, too. So after you use it, flip it around (your daughter may need help with this if her back hurts). Unfortunately, it's bulky, too. We just hide it in the back of the tub.