Bike Tools Etc. to support the bike industry. McMaster-Carr, Bearings Inc. or Grainger, which are industrial supply houses. Most auto supply houses if one is close by. Grade 200 is adequate but Grade 25 is better and the cost difference is very minor. Rust is usually not a problem and like bikeman715, I've just left them in the original plastic bags in an unheated garage for years and never had a rust problem. A spray with WD-40 or light oil is fine but probably not needed.

Finally, remember bicycles are very low-demand applications for bearings. The rotational speeds are low, the loads very modest and the temperature range laughable compared to many industrial applications. So, buy decent bearings and don't agonize over the unnecessary.