Hi Arrielle,
While I’m even less familiar with the bedframe than Jeff … as far as I know Leggett & Platt only supplies the linked spring or the polydeck options that Jeff mentioned (see here). I suspect that the comment in the reviews was probably referring to either a slatted bunkie board (see post #4 here and post #4 here for more about bunkie boards) or perhaps a slat roll such as this that they purchased separately (although if you were to use something like this you would need to make sure that they fit well and that they would stay secure on the bedframe without shifting).
Phoenix