Hi
As per my understanding from u post ur running BGP with Sprint from all ur locations ,do clarify whether ur running any routing protocols like ospf or eigrp in between u r locations or simple static routes.
if OSPF is already ther in place u can distribute the default info thru the ospf to the other locations.
From Site B again u can announce the routes whtever ur receiving frm ur SP thru IBGP.
During normal conditions when the Internet Link in Site A is up all the locations will reach the outside world thru tht particular site taking the route via ospf.
when tht internet link is down they can again go out via the IBGP route avl from site B.
Hope this clarifies some of ur queries to start with.
regds
