Hi Andy. Welcome to justanswer.
There are bands included in the box. Have you tried using them?
I am sorry to tell you but Jaybird does not sell any bands separately.
Now you have two options left.
Either you can buy a new one of L/XL size (which I won't advice you) or you can buy a used tracker of L/XL size in which bigger bands will be included that you can use. I won't advice buying a new one because this has been discontinued by jaybird and there are many alternate trackers available which costs less than this and have better facilities.
Now tell me Andy what would you like to do so that I can assist you further.
Hi, just checking in. How did things turn out?