ERF Group 1 seat to fit in an old Astra?

[Cydonia]

Hi, I have a 6.5 month old DS who hopefully has a few months left in his infant carrier, but am just starting to look at what comes next.
From what I've read on here I would like to keep him rear facing for a bit longer, so can anyone recommend me a seat?!
It needs to fit into a Y reg Astra estate and an 02 reg Mondeo hatchback. Also ideally not too expensive, or should I say would like value for money, ie a seat that will last a good few years! And where can I buy it from, as they don't seem widely available?
Sorry for the silly questions, a bit clueless!

My friend has a two way elite in her y reg astra because of the length of the buckles apparently. Look on in car safety website or call them for advice, I found them very helpful :)

Thanks, I will do. As I will probably have to order one online I don't want to end up with one that won't fit in my car!