Although I'm not a parent myself (at least that I know of!) my almost 3 year old nephew and 7 month old niece have numerous car seats some of which can be transferred amongst the families cars. I haven't really paid attention to the choosing of them, all I know is that where possible, various features were evaluated, safety records checked and a test sit where possible to see if they suited the child was done. I guess you could buy directly online without any test sits etc too but if you find it's not quite suitable, then it's the hassle of returns after you've gone out and purchased another etc
Check Mothercare stores to test sit your child into to begin your evaluations. I'm sure there's other places you could try but again, I've not paid much attention as to where.