What are some of the strength building exercises for Bass players, especially for the left palm/fingers?
Everything Dr Mayhem mentioned is fine. If you can (and like the idea) climbing can sure help in strenghtening your hands.
Apart from that, I'd do a lot of spider excercises, especially on the upper part of the neck.
This means playing 1-2-3-4 (i.e. index on the first fret, then medium on the second, then ring finger on the third and finally little finger on the fourth) and back on each string. This is the simplest, but you should try all the combinations, i.e. 1-3-2-4, 1-2-4-3, 1-4-2-3, 1-4-3-2, 2-1-3-4, and so on.
One of the best exercises for fingers is to use a FingerGrip type exerciser like this one:
And I would also recommend using a tabletop if you are sitting at work, pressing your fingers hard against the surface.
Carrying bags of shopping is also useful - try heavy bags and just using one or two fingers - I use this for strengthening my fingers for climbing, along with chin ups.
Have a read of this related question - guitarist focused, rather than bass, but it's broadly similar.
Bass Fitness is an exercise book in the likes of the "spider exercises", mentioned here.
I (try to) use it on a regular basis and recommend it to a fellow bassist. ;-)