Hi @gentaharuyama
ifixit is about getting advice on how to repair things yourself.
Here's the ifixit [guide|162365] guide that shows how to replace the battery in the earbuds.
Unfortunately the images of the battery are too blurry even when zoomed in to make out the model number of the battery.
Once you have opened the earbuds to access the battery, note the battery model number which is stamped on the battery and then search online using that information to find suppliers that suit you best.
If doing this yourself seems too daunting, contact a reputable, professional electronics repair service and ask for a quote.