Is your son over 25 lbs? If so, we like using the "Tote N Go" car seat for trips where we'll only use taxis. I wouldn't use it if you plan to drive extensively - it's safe, but not as comfortable as a regular car seat. Also, we practiced installing it at home before we used it for the first time, as getting the hang of all the straps can be a little tricky when you're in a rush. That said, it is very light (less than 4 lbs) and small - it fits in a backpack or duffle bag.

A quick web search shows that you can buy one for $30 or so: http://www.babyage.com/products/22-9...e_car_seat.htm
but there may be stores where it's cheaper.

Cheers!