I personally use Tom Ridenour's Thumb Saddle.
A reader has just sent this link to the Ton Kooiman Etude2 adjustable thumb rest. It looks like it would be at least as good or even better than the Ridenour Thumb Saddle.
More and more clarinet players are finding that a neck strap will help. This is particularly helpful for young children with small hands who are starting to play clarinet. There are several different instructions on YouTube for how to attach a common saxophone neck strap to a normal clarinet thumb rest. Here is one: