If you buy a sepia toning kit, both the bleach and the developer will be included. Be aware that there are basically two types of sepia toner: ones that will stink the house out and those that do not. Use the former outside, but the latter has no smell and can be used inside. In either case, the bleach has no objectionable smell.
You have two options when toning: tone to completion - i.e. until the image has all but disappeared, wash and then put in the toner... Or, only bleach part of the way, until the highlights have gone, but the shadows still remain. This gives a more subtle effect, depending on how long you leave it in the bleach.
If you want to know the chemistry and a shed-load more besides, get hold of Tim Rudman's toning book.
Have fun, Bob.