How has she outgrown it? in height or weight? They are supposed to be in infant seats until they are at least 9 months - if it is just that her legs are too long for it they say that this is fine, that you only need to worry if their heads are above the top of the seat.
She is too young to go into the next stage car seat yet, even if she weighs over the 20lb mimimum, so your best bet would be to look at a 0-4 car seat, that you can have rear facing for now and then turn around when she gets older, these are bigger than infant carriers so she shouldn't grow out of it for a while yet. I don't know anything about these I am afraid, but I know that Gem and Leland has one - so you could PM her and ask which one.