The best (but also the most expensive) is an original Acer battery. However, you'll find many shops (especially online) that offer third-party batteries that are supposed to be compatible. Be sure you can return (and get your money back) it if it isn't.
Replacing is simple: take out the old one and put the new one in. Usually, you don't even need a screwdriver. See your user manual for details. And you won't lose anything.
By the way, having the laptop on mains power as much as possible, is the recommended procedure to give the battery the longest life.