Err… In reading your question, I get the sense you’re thinking the
granules are glued onto the surface. They’re actually fused on
metallically in one way or another. The glue’s just used to hold them
in place during the fusing process, whatever that may be. (There are
a couple of ways to do it.) The glue burns away during the fusing,
and has no further role beyond that point.
To answer your question, hide glue can be had from larger hardware
stores that stock fine carpentry supplies. “serious” woodworkers use
it from time to time. I know Elmer’s makes it in a squeeze bottle
that looks just like, (and sells near) the titebond wood glue.
You can also use gum tragacanth, and I personally use Elmer’s white
glue cut about 25/75 with spit. (Spit’s a surprisingly good
surfactant.) Any basic organic glue will probably work just fine. The
whole point of the glue is first to hold the balls in place prior to
firing, and then to supply a bit of carbon during the firing.