Your question is very interesting because it outlines the differences between gun and knife marketing.
Guns tend to remain unchanged for long periods of time (years) where knives have a model life of 6 months or so before they are upgraded or replaced. For this reason you seldom see aftermarket modifications appearing on the market.
Knife handle replacements are some of those items that do not.
About the only way to get replacement handles or special sheaths is to contact a knifemaker who will replace the handles and mould a kydex sheath for you. This can take a bit of searching on the knife forums but you can usually find a knifemaker near you to do the job.
I would take the job on myself but I am located near Toronto Canada a bit too far.