You can order it online. There are number of online stores of various popular brands. If you want do to offline shopping then also Google will help you in finding a store in your vicinity from where you can buy wrist bands. http://streetlegaltshirts.com/movies.html
I am very sorry to hear that news, just dreadful :>(
I was trying to find out who actually makes this watch, but there is no data. Normally you send to the maker and they repair it. My late wife bought me a Seiko, and although I live in Canada, I send it to the UK where it was purchased and where the service centre is for servicing. you could ask Nike who was contracted to produce it? My guess is Casio, because it has the Casio look to it.
One thought I have is that the battery that was put in it was the wrong type or it was inadvertently replaced with a battery out of another watch that came in for repair. I would have another battery put in it and see what happens. It is actually a collectible watch.
https://www.google.com/search?q=Who+makes+Nike+WC0045
There are a few ideas which battery is the correct one
https://www.google.com/search?q=nike+wc0045+watch+battery+replacement
....
