Where will I find a replacement bolt and nut for this gem? This one was there when I got these and it's kind of big and clumsy... any ideas?

Good question, since the original bolt & nut were probably not steel....

Do they make brass carriage bolts?

Hmm, yes, google says several places sell them, so, grab a brass carriage bolt, half thichness nut, and your trusty file...
(You will have to file off the square part under the head..)

Hadn't thought of that.
Sounds like a plan. Filing is fun.

You might be able to find a replacement at NSK Berylco.  They manufacture beryllium tools and have a plant in
Sweetwater, TN.  A new pair of pliers such as you have will cost around $80.00 or more so it would be worth fixing yours.  If interested here is their email:  marketing@ngkmetals.com

Good luck with the repair and let us know how you make out.

Thanks! I will check it out.