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You might find one for free on sites like KVR Audio.
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If WaveLab 6 supports plugins (I've never used it), see if there is a compatible plugin available. The Wikipedia page on de-essing explains at a fairly technical level a couple of techniques for doing this, but you don't necessarily have to rig it up yourself, as there are several de-essing devices available, both in hardware and software. The process for doing this is known as de-essing. It can be very distracting to listen to, and "lessening the impact" is exactly what you want to be doing. That said, the "ess" and "shh" sounds exhibit quite a lot of sibilance, which tends to show up in recordings, like you said, as a burst of volume and noise.
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If you have a lisp, then it's a natural part of your speaking voice and you won't be able to get rid of it entirely through processing (and you shouldn't, in my opinion).
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