Or you can make your own:
One good thing about doing it this way is that the material is thin and you can experiment by trying a different number of layers. Some bike tubes have ribbed areas, so you can make a ribbed or smooth grip.
I got a couple of tubes free - went to a local bike shop and asked for any bad ones they were throwing away. http://www.ktrange.com/articles/a7/a7.html
Magazine Finger Extension Base Plates for the Kel-Tec P-3AT:
The NAA style extensions must be modified slightly.
The Bersa (smooth) and Scherer (textured) style extensions need no modifications.
Another option is the Kel-Tec 1-Round Magazine Extension which extends the magazine and also allows loading an extra round.