Shimano ball bearings 1/4" cost $2 for a pack of 18 balls. Part number Y00091310
You can find them almost in any big online retailer. They are steel, not stainless.
I got my balls from LBS because I needed them fast. I think they charged me 10 cents for each ball. They were unknown/unbranded.
Also, the LBS had a pair of double ended cone wrenches for around $9 (big brand LBS).