What's the best substitute for chewing fingers?

ARK's Krypto-Bite, Bite Saber, and/or Baseball Bat are usually best for that (the reason being they're long, slender, and smooth, much like a finger - so you're matching the chew to what the person already likes to chew on).

Most of our chew tools (these included) come in 3 color-coded toughness levels.  The softest/standard level is usually the one to start with here.  *Unless* the individual has a very strong need to chew and is chewing through other things as well (chewing holes in shirts, chewing through pencils, etc.) in which case the tougher XT and XXT options would most likely hold up to that better.