I have always bought mine from here...
http://www.wholesalersusainc.com/gla...bles-c-58.html
they come in a variety of sizes - if you find thier MOLDED ones they are excellent - very clear and perfectly round and very easy to use a punch
Large = 1.5"
the extra Large and Jumbo should also be perfectly round...but for making the glass gem magnets the best size is the large MOLDED ones...
They also sell magnets there too...
Make sure when you are attaching the image to the gen you do so with some sort of good glue - I did a bunch last year using permanent xyron adhesive but with age the glass gems are starting to fall from the paper circles...
Crystal effects is a good permanent thing to use