Nimetic

There was once something called Power Putty, which was good for working the fingers.

Apart from that, there are silicone balls in fitness shops (that I've seen) - and failing that, when I was a young punk with an interest in martial arts, I used to just squeeze an old tennis ball as I walked from A to B. This has the advantage of not looking weird. You just bounce it if anyone takes an interest.

But I think these mainly develop strength.

For endurance.... I can recommend some tough and painful piano exercises... : >

Oh yeah. Use one finger at a time... not the lot together. Otherwise it's too easy.