The extenders are on the parts list. They are a bit hard to find if you don't get the name right.

Call S&W at 1-800-331-0852. You want part no. 242120000 Grip Plus 2 ButtPlate.
I think they cost $9 or $10 plus shipping.
You might order a few and some springs to even out the shipping cost.
They may or may not be on backorder. Mine were, but were shipped out shortly after I ordered them a couple of months ago.

I actually ordered them as an experiment for a PPC extended magwell & 59 series 20 round mags, but I will use them on the 15 round mags as well.

The 3913 grip is just a bit slim for me as well even with Hogues. The CS9 fits a little better. I have a beater 6906 on the way to see if that will work for me as well.