Hi charlotte,
Welcome to the Mattress Forum! 
I would recommend purchasing a finished mattress versus a mattress and a topper, as purchasing the items separately tends to further complicate the situation (see my thoughts in post #2 here).
As for a spring/latex mattress (hybrid), while not a complete list, there are quite a few providers in post #2 here, and that should give you a good start on your search.
Safe travels!
Phoenix