Kind of late to answer this, but I thought I'd give you my two cents since no one has replied...
We have a "fit disc" that I bought on Amazon. The ones they sell on the special needs websites are basically identical but more costly. Ours has ribs on one side and nubs on the other, so my kiddo can chose whether he wants smooth or bumpy. We have one on his seat in our kitchen and he uses one at school too. He has moderately low tone in his core and can be kinda floppy when he's sitting. The disc has helped his slouch a LOT and also improves his concentration. The only caution I have is that, if you use the disc on a normal-height chair (as opposed to a child-sized chair), you'll want to make sure your daughter has something she can put under her feet. The disc will boost her up a few inches and, unless she's VERY tall, her feet will probably dangle in the air. This makes sitting on the disc really tricky and unstable. I'm short and tried using the disc myself and couldn't because it was so wobbly. My son has a Tripp Trapp chair, which has a built-in platform for his feet. If you don't have something similar, you'll probably need a footstool.
We got one for my ds because his OT recommended it. He focused much more at OT with it. He did not like it at school. Kind of felt singled out, so only used it when he was in special ed room during 4th grade year. He did not use it in 5th at his request and did not need it. He is in 6th now (middle school) and I think it would be to difficult to keep track of because he travels between many classrooms. Ours were like the one described by Monzie, but I bought through a medical supply company. I liked using one while I worked because it made my back feel better. I ened up loaning it to my pregnant friend and I think it moved out of state with her. I really disliked the smell of the plastic to the point that I do not miss the back support.