There are several guys that do custom handle jobs. My first recommendation is you buy the knife you think you want to upgrade and use it for a while and see if you love it. If you do then get it handled. If not, you won't put your money into a knife that isn't a favorite and you can try something else.
It depends on who does the job. There are several guys on this forum that do them. Adam Marr, Tim Johnson, Matt Delosso and Matt Stephens all do them. You can PM any of them and they'll respond.
You can see their work in their subforums.
Sometimes we off these off the shelf with no wait. If you check Adam Marr's page later today I'll have one up that is a 240mm that he did recently. They don't usually last long.http://www.chefknivestogo.com/adam-marr-knives.html