Me and my husband made the hooves together. They are 4" heel boots with the heels ripped off and PVC pipe stilts attached to the bottom of the toe. A metal piece stabilizes the shoe above the pipe and cardboard forms the shape of the hoof itself. They made me 14 inches taller.

I could walk in them just fine, but they were not for walking or standing in for long periods. They took some leg muscle to maintain balance and the pressure hurt the balls of my feet after a while. I was also sensitive to uneven paving and deep carpet. I had a secondary 7" pair for walking distances.

I also made the horns, ear, tenticles, tail, armor, fangs.

I have started a collection of hoof construction photos and info here: http://www.cosplay.com/gallery/162790/
My construction technique is ideal for dramatically increased height, exposed calf and thigh silhouette, and slender, disproportionately long legs. They are also fairly upright, with the stilts coming close to being perpendicular to the ground. If this doesn't fit your costume, I would recommend looking at some other options:

http://www.transform.to/~mwbard/stuff/thehorsetailor/indexcomfyhooves.htm
Good for bulky digitigrade legs that are proportional in length to the rest of the body. Most comfortable option. Could be used with a heeled shoe or flat stilt (ideal for Tauren) to be taller.

http://gryphern.googlepages.com/
Good for slightly bulky, more angular digitigrade legs. Intermediate height increase. Requires braces on the legs. Probably better than mine for people unaccustomed to walking in heels.