About 20 bucks on Amazon.
How does the NB4L battery work? Like most batteries, this battery works by an internal charge storing energy for later use in electronic devices. Batteries that are often subjected to cold temperatures tend to not last as long, mainly because this is a poor condition to keep these types of products in.
It is unlikely that the NB4L battery will work for your lawnmower but it really depends on your mower. Most of these batteries are used for small electronics like cell phones.
You can purchase this type of battery at most online electronic retailers. This includes Amazon, eBay, Best Buy, and Newegg. All of these retailers should have what you're looking for.
They do sell them online. But I would actually just go to my cell phone provider and have it replaced. Some places offer it at no cost to you if you signed up for insurance. If you want to skip that you can try Amazon.com or Ebay.com