I'm looking for replacement bearings for a Shimano M760 XT rear hub. According to Shimano's technical documents website, it uses 1/4" bearings, nine per side. The lowest price I found for bicycle bearings was 25 for $4.80 at JensonUSA.
Edit: Price ($4.24 to $4.37, so 13 cents in two years) and link were updated in July, 2013.
Regarding the grades of bearings, according to Wikipedia, inch-size ball bearings are graded from 1000 to 3, where smaller numbers indicate higher precision. It appears that Grade 25 is commonly used on higher end bicycle components, with Grade 300 on some low-cost parts. Different materials are available, such as stainless steel. Harder bearing materials should not be used, as this will cause the bearing races to wear out rather than the cheap and easily replaced ball bearings. Also be aware that steel balls are not the same as ball bearings, are much harder, and should be avoided for the same reason.