Ball bearings, aka BBs, are just little metal balls. You can buy bearings from bicycle part distributors or from local bearing companies.
The easiest way to determine the bearing size is by using a special ruler. The most common sizes are:
- 1/4" Found in Rear Hubs and Bottom Brackets
- 3/16" Found in Front Hubs
Less common sizes:
Bearings come in different Grades, the important thing to remember is quality and cost increase as the grade number gets lower, so 100 grade is better than 1000 grade. Grade 25 is the best you can buy, but you can use bearings all the way up to grade 800.
NOTE: Avoid mixing different kinds of bearings as they are made in batches, and bearings from the same batch work better together. It may be tempting to consolidate bearing containers when one gets low, but it is better to just let them run out.
Starting with the highest quality and most expensive:
- Stainless Steel
- Chrominum (Chrome)