The softest inserts I know of is the soft white Contour Power Grips. I have tried many different grips, but since I have used these for years no other grip is as soft or comfortable. Even grips other companies claim are their "soft" grips are no comparison. Just for information, there used to be some silicone grips around that were softer, but had to be glued in using commercial silicone and had to cure for something like 48 hours before use. Oh, they wore out almost that fast too.

No idea about where to buy on the internet, since I get mine from the local proshop through the local bowling supply company.

I use Vise because i like them and my proshop guy is on staff with them so thats what he stocks.  He will order you anything else though if you just ask.  Back to your question which are the softest not vise, Turbo grips are definitely softer than vise and I don't know about contour or any other brands and I beleive vise and turbo are the two biggest companys.  Also a point to consider the softer the grip the shorter the life span and quicker you have to replace them.  Vise aren't the softest but they are comfortable by all means and last a long time.
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I should just quit bowling, oh wait I already tried that.
Now that I am back and my Saws are sharp again, I am ready to cut some wood.