If you have a frame with wheel mounts at 90 degrees to each other (45 degrees from normal), you could use regular omni wheels:
(
http://www.andymark.com/ProductDetai...ctCode=am-0013)
Mecanum wheels in that configuration would not be omnidirectional. Rotating the front right and back left wheels 15 degrees counterclockwise and the front left and back right wheels 15 degrees clockwise would provide more power in the forwards direction (I think) while retaining some omnidirectional ability.