K.S.
E.- There are second-hand stores where you may find a car seat. You definitely want to get a new one if your son has outgrown the recommended sizes, but you will have to weigh whether you will need one if you don't have a car. Do you ever go anywhere with friends or family who have a car? Or what if you had an emergency and needed to take him in a vehicle? As for the air travel- you only need a car seat if you are paying for a seat for him- he is young enough that you could have him on your lap and save that expense. If it's a long flight and you want him to sleep and he would do that better in the seat, then I say get one. You can always check your stroller plane-side so you have it in the airports. I am thinking if you take the old infant seat on the plane he will be uncomfortable becuase his feet will be hitting the seat- that's why the height/length recommendation is there- the seat has to be rear-facing and the child's feet will hit the seat and it would be very dangerous in an accident because there legs would be jammed against the back seat on impact. My ped. also told me in the past that a taller child in an infant could possibly push with their legs hard enough to flip the seat! Yikes!
Not sure where you live- but you can look for a Children's Orchard (there are many in MA) at www.childrensorchard.com
There are also a few other places I know of- I love "Kid to Kid" on Rt 9 in Natick and just found "BeanSprouts" in Stoughton. I believe there is a place called "Keeping Pace with Multiple Miracles" in either Randolph or Brockton but I have not been there yet.
Good luck!