But, I did find this….

I have had good luck installing my "mini-car-seat" in American taxis. I think it is actually called a "Travel Vest." I bought it brand new for about $40 at Baby World on College Ave. It amounts to a rigid plastic board with a five-point restraint on it, which is belted into the car’s seat belt. It meets the federal safety standards and goes up to a 40-pound child. It installs in about three minutes–probably less the more you use it. It isn’t super comfortable, because it doesn’t have the nice reclined chair-shell that typical car seats have, but for a short ride while she is awake, my 18-month-old daughter is fine with it. Also, because it is very small, it can easily fit into a tote bag. Hope this helps. Meg
References :
http://parents.berkeley.edu/recommend/where2buy/carseats/taxi.html