Here are a few videos of amazing contemporary art glass marbles. It is difficult to do justice to these without having them "in hand" but thanks to higher resolution videos you can get a pretty good taste. Marbles of this quality can cost several hundred dollars, which considering their beauty of color and depth of construction, may be among the best deals in the art world.
The first is a marble made by Yoshinori Kondo of Japan.
An aptly named Universe Marble, by Gateson Recko:
This marble is a collaboration between Mike Gong (USA) and Rose Roads (Japan):
Finally here's a huge marble by Kenan Tiemeyer that appears to have embedded opal in it:
Where do you buy these? Vendors at marble shows often have them. Online you can buy many from Glass ORBits.