Reddit reviews 500 Piece Assorted Loose Bicycle Bearing Balls 1/8", 5/32", 3/16" 7/32" & 1/4"

Most hardware stores sell bearings in the specialty hardware section. Or something like this: http://www.amazon.com/dp/B018B7D05K

> A 1/4" bearing from a "bike specific" company is the exact same thing as a 1/4" bearing from any other company

As long as they're not a terrible grade... Grade 25 is typical for bike use these days and they're cheap enough not to bother with something like a grade 300. There are still grade 300's being sold by bike companies, but like you said, they're more expensive (even though they're lower quality).

Surprisingly cheap. Here is a grab bag of 500 bearings of assorted sizes between 1/8 and 1/4" for $8.

It's easier than that, even:

Just buy an assortment of bearings. They're cheap. Then when you take it apart, you will definitely have the right size, just match the old ones to the new ones... Which will be 1/4", since this is an 80's Japanese bottom bracket and will be totally standard. ;)

(edit: And then you'll have bearings for your hubs and headset too!)

You should definitely replace any missing ball bearings, they are pretty cheap and will make the bearings smoother than they would be with some missing. Here are some bearings i got in the past that are good, but you can always just get the individual sizes you need instead.

You can get an assortment of bearings on amazon. I got these when i serviced my bike: https://www.amazon.com/gp/product/B018B7D05K/ref=ppx_yo_dt_b_asin_title_o09_s00?ie=UTF8&psc=1

I would buy something like this assortment, pick the one that's the right size, put some grease on 'em, and get on with life. The extras will be useful for something, eventually.

edit: typically, you fill the race up with bearings, and then remove one. So if it takes 10 to fill the race with balls, use 9.

Are you sure? Most BB bearings are about 1/4 in.

In any case, amazon sells them for cheap

Ex: https://www.amazon.ca/BC-Precision-BCBIKE5-500-Piece-Assorted/dp/B018B7D05K/