According to the Lesser Key of Solomon, the "nine herbs of purification" are lavender, fennel, vervain, sage, valerian, basil, rosemary, mint and hyssop. Of course, in the KOS, they are also bound together after being plucked in the morning dew by a virtuous maiden, so it's not like you really have to be a stickler for details these days. Anyway, in theory, you could substitute any of these for lavender in a purification bath. A good rule of thumb in herbal substitutions is to follow the doctrine of signatures, rather than swapping out herbs based on color or appearance.
Watch out for the fennel, though. It makes your skin smell like licorice.