I don't have a child, so not sure if this is a good seat (but it says it's original porsche on the ad). On the sf craigslist, there is someone selling a child seat for 37-100lbs for $75.00 if someone is looking for one. Doesn't come with the airbag deactivation bar (Just the seat and Buckle), but it sounds pretty inexpensive.
Original Porsche child seat 37 - 100 lb