C.T.
I could not find one that had a base.
My suggestion is get two car seats. They do not have to be expensive. I think it would be difficult having to switch car seats b/c they are so heavy and you have to spend some time getting them adjusted and just right.
My daughter is 16 months old so I just went thru the whole selection thing. I did some intensive research on carseats.
I was torn between the Britax Boulevard ($350) and Evenflo Triumph LX ($140). I ended up getting both. Since my car is the car we travel in, I got the Britax. My husband takes her to and from school and other places around Houston so we got him the Evenflo. Both seats were rated very well in safety on my sites.