Help on child car seats!?

I'm trying to find out what child car seats are usually made out of? It might sound like a strange question but I brought a car seat from Argos a few weeks ago that turned out to be made from polystyrene & it pretty much came apart in my hands while my daughter was travelling in it. I was very worried about the safety of my daughter that I no longer use the car seat. Anyone know of any that are made of something a bit stronger? I don't have a lot of money so I can't afford "the best" ones. We don't drive at the moment so we only need it for when we are in taxis or family cars. Thank you :-)