http://www.toyrepairs.co.uk/toyeyes.htm
http://www.hauntershangout.com/home/easyeyes.asp
http://www.glasseyes.com/
http://www.bjcraftsupplies.com/dolls/eyes-animal01.asp
http://www.softglasseyes.com/

are interesting links for eyes.  The best tend to be rather expensive.
You might try using a glass disc and painting the back of it in reverse - i.e. the pupil first, then the iris.  I've used this for fish and it can look quite effective.
DavidO

It is possible to make realistic-looking eyes in sculptures without glass eyes.

One way is to sculpt the face with open eye sockets (eyelids but no eyeball), like dollmakers do with porcelain dolls.  Then you make a pair of eyeballs and insert them behind the eye sockets.  You must realize that eyeballs are much larger than they appear, as we see only the front surface, not the entire ball.

This article shows some drawings of eyes, and photos of making eyes with polymer clay:  http://www.squidoo.com/sculpting-eyes

The eyeballs can be made from polymer clay (if you have access to it - some brands are Sculpey, Fimo, Cernit) which can be baked in a household oven, large clay, glass or wooden beads, nuts or pods, or toy balls that have been painted to look like eyeballs, etc.  If you know someone who fires clay, you can use clay.

If you have to make your own, you can also make half-spheres, but sometimes it is easier to find something round than to make it.  Talking to a dollmaker, she admitted that sometimes she has to resize her entire doll to fit the eyes she made from found objects.

To get the lifelike effect, you need to do some careful painting, and then coat the dry paint with a clear glaze, like glossy varnish or clear nail enamel to give it that realistic shine.

Sue

The eyes that are cast in that tutorial remind me of those clear glass nuggets that you buy from florist shops to put in vases with artificial flowers. There is another option for anyone who doesn't want to mould their own, although you would be more limited with size of course.