K.I.
My daughter had outgrown her infant carrier by 10 WEEKS (she was well over 20+ lbs)!!! I luckily have an older son and we just passed down his old one but I can tell you that CRAIGS LIST is a GREAT spot for this sort of thing and they typically (for this area) have quite a bit of Britax, Also try a JBF (Just Between Friends)sale (consignment), there will be one this weekend in the Midcities and several more over the next couple of months throughout the metroplex- Just go to JBFsale.com
HTH!!!