One of the Fit forums says Bosch Mega Power 44b19l-s. I would price one at Honda dealer and then check around and see if a Bosch dealer can supply one and at what price.
Another forum says that a Group 51R will fit and a couple of fooks have done this for 07-08.
"Group 51R will drop right in (though it is slightly longer), and most batteries in that size are approx. 500 CCA. The Fit battery is group 151R and nobody makes it but Yuasa for Honda. Just get the 51R and be happy that it's slightly more powerful than the original.
I would also think that this battery should still be under a full or pro-rated battery warranty that the Honda dealer would be able to honor.