What exactly do the vet say? I was under the impression that there is no surgical fix for spay incontinence? If this happened before the spay, then that, along with a possible surgery, points to something other than estrogen issue? Not sure that more can be said without further information.
I haven't read the above link, but anything from food allergies to phyical abnormalities in the vagine and bladder can cause incontinence. Cystitis from high urinary specific gravity - all sort of issues are possible.