Do you have a pull-up bar (or something that could act like one) at work? I have been solely doing calisthenics for the past years and my grip improved tremendously from just one exercise: hanging.
Start by hanging a total of 2 minutes and gradually increase the time to hang even longer.
Example:
Week 1: 6 sets of 20s = 2 minutes
Week 2: 4 sets of 30s = 2 minutes
Week 3: 5 sets of 30s = 2:30 minutes
And so on.
Once you are comfortable, start doing 1-arm hangs:
Week 1: 4 sets of 30s L/4 sets of 30s right = 2 minutes on each side
Week 2: 4 sets of 35s L/4 sets of 35s right = 2:20 minutes on each side
Week 3: 6 sets of 30s L/6 sets of 30s right = 3 minutes on each side
Not everyone may agree with me on this one, but I feel like grip strength doesn't necessarily have to be worked on directly. You work on it while doing pull-ups, Romanian deadlifts, etc.
And if you can get your hands on a pair of gymnastics rings (not expensive; about $20 or so), doing pulling exercises on those, like pull-ups, inverted rows, and
rings scapula retraction, will help your grip tremendously.
And since I've mentioned it...
Doing rings scapula retraction for 3-4 sets of 5-8 reps, holding each repetition at the top for 3 seconds, will kill your forearms and help in other areas too.
If you have any questions let me know.