laptop questions

i just bought a laptop used on ebay, and when it arrived, not suprisingly the battery was dead.  so i have a few questions now...

what should i do about getting a new battery?  the batteries i see for sale are really expensive.  do i actually have to pay that much to get a working battery?

if i get a car charger for my laptop, how long can i sit in my parked car and use my laptop before it kills my car battery?