You should be able to find some colored lenses that provide a decent bright blue. Ciba (http://www.cibavision.com
) offers some fairly natural colored lenses (look at their "FreshLook" and "Other Lenses" sections). You didn't say what your current eye color is, but if they're dark, it may be hard to reach a good natural-looking blue... then again, Cloud's eyes aren't supposed to look entirely natural since they're rather artificially enhanced by mako anyway.
Just remember that you will need to visit an optometrist's office to get any kind of colored lenses, even if they aren't in a corrective prescription; the lenses still need to be "fitted" and require a prescription to purchase. I've seen people show up the day before Halloween at Wal-Mart's vision center expecting to buy colored lenses and have to leave unhappy.