newborn insert car seat?

kasey
Wondering whether or not any stores sell generic infant inserts for car seats? Just any old thing to fit in the car seat. We plan to use my 11 month olds car seat she's in now for the baby and get her a new one. 
But unfortunately, I can't seem to find where I placed the newborn insert! I looked online and really didn't have any luck. Anybody know of anything?