I was a potter for 20-some years before becoming a jeweler, and still
do ceramics to a limited degree. You can find most of the suppliers
online by searching for "ceramics supplies". Great Lakes Pottery
Supply, A.R.T. Studios, Soldner, and others all sell glazes. You will
find that commercial glazes are available essentially in three firing
ranges, with low-fired glazes the most vivid, and higher, more
subtle. Most serious ceramic people mix their own, and there are many
excellent books on the subject, particularly those by Daniel Rhodes.
I have never encountered powdered gems as glaze ingredients per se,
but glazes are all made from mined minerals, and the "ingredients"
would be similar. Much more convenient to buy them already ground to
200 mesh or so.
I must say, as someone who spent a large chunk of her life
more-or-less mastering this medium, that I hope you don't have the
idea that you can become a potter as a sideline, in a bit of free
time. It is as demanding an art form as any other, and more than
In any case, good luck to you in all things!