The biggest problem with buying omni wheels is that there are pretty expensive, so therefore making them is the best bet. There are so many ways of making them that I won't go into, but a really cool model of how to do it, is Wildstang's design from last year. I don't know if you remeber it or not but we were thinking about modeling our omni's after but decided against it in the end. And lastely if you actually decide to buy your omni's try
www.acroname.com