Originally Posted by dedhed

You buy bearings based on the hub design, not the brand, year, or model of bike

Most bicycle cup & cone hubs will use 3/16" or 1/4" balls depending on manufacturer and/or front/rear. You buy loose chrome steel balls from a bearing supply or LBS in the correct size and replace.

Sealed bearing hubs will use standard cartridge bearings (for example a 6800 2RS) that you read the numbers off of and purchase accordingly from either a bearing supplier or LBS.